Variations of Okhotsk Sea Oxygen Minimum Zone: Comparison of Foraminiferal and Sedimentological Records for Latest MIS 12–11C and Latest MIS 2–1

Marine Micropaleontology 121 (2015) 52–69

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Marine Micropaleontology

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Variations of Okhotsk Sea oxygen minimum zone: Comparison of foraminiferal and sedimentological records for latest MIS 12–11c and latest MIS 2–1

Natalia Bubenshchikova a,⁎,DirkNürnbergb,RalfTiedemannc a P.P. Shirshov Institute of Oceanology, Nakhimovski pr. 36, Moscow 117997, Russia b GEOMAR, Helmholtz-Zentrum für Ozeanforschung Kiel, Wischhofstr. 1–3, D-24148 Kiel, Germany c Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Am Alten Hafen 26, D-27568, Bremerhaven, Germany article info abstract

Article history: Benthic foraminiferal assemblage compositions, foraminiferal and sedimentological proxies were analyzed in Received 9 October 2014 core MD01-2415 from the northern slope of the Okhotsk Sea to compare variations of productivity and oxygen Received in revised form 21 September 2015 minimum zone (OMZ) intensity during the latest marine isotope stage (MIS) 12–11c and latest MIS 2–1. The ben- Accepted 30 September 2015 thic assemblages reveal close similarity between the two climatic cycles. The absence of benthic assemblages Available online 8 October 2015 during the latest MIS 12 and the presence of the low-productivity Angulogerina angulosa assemblage during the latest MIS 2 suggest the disappearance of the OMZ. This regime was related to almost perennial ice cover Keywords: Benthic foraminifera with periods of active ice rafting during the latest MIS 12, while it was attributed to prolonged seasonal ice Organic matter flux cover, low surface productivity and enhanced formation of well-oxygenated water masses in the Okhotsk Sea Oxygen minimum zone during the latest MIS 2. In deglacial times, the OMZ gradually intensified, as evidenced by the high- Downslope supply productivity Uvigerina akitaensis assemblage during the early termination (T) TV and early TI and the low Carbonate dissolution events oxygen-tolerant Bolivina spissa assemblage during the late TV and late TI. The orbitally forced similar rises of Okhotsk Sea the global sea level during TV and TI caused a large offshore supply of organic matter. Synchronously, nutrients from the melting sea ice and shelf erosion promoted high surface (mainly carbonate) productivity. As a result, a high and sustained flux of particulate, degraded and refractory organic matter enhanced oxygen consumption in bottom waters and sediments, leading to the similar gradual OMZ intensifications. The B. spissa assemblage also points to expansion of oxygen-depleted water mass from the North Pacific into the Okhotsk Sea, fostering the OMZ intensifications. The phytodetritus-related Islandiella norcrossi assemblage indicates weakening of the OMZ during full interglacial times of MIS 11c and MIS 1. After stabilization of the global sea level, nutrients were mainly delivered by regional upwelling and fluvial discharge, favoring increased biogenic opal and carbonate production in the surface water, similar to the present. In this way, moderate to high (although less than deglacial) and pulsed flux of predominantly particulate organic matter caused the weakening of the oxygen consumption and OMZ. Notably, during MIS 11c, the benthic assemblage with the dominance of the taxa with dissolution-resistant tests, such as Miliammina herzensteini, Karreriella baccata and Martinottiella communis,reflects carbonate dissolution events in sediments. These events might have been resulted from an interruption of the local surface carbonate production and inflow of more carbonate-corrosive water masses from the North Pacific driven by a drawdown of the global ocean carbonate saturation. © 2015 Elsevier B.V. All rights reserved.

1. Introduction large amount of nutrients introduced by seasonal sea ice melting, fluvial discharge and local upwelling. The presence of an oxygen minimum Variations in the Okhotsk Sea primary production and formation of zone (OMZ) at low intermediate depths in the Okhotsk Sea reflects intermediate water influence global climate via the contribution to at- the balance of the high primary production and ventilation controlled mospheric CO2 concentrations and ventilation of the intermediate by both the outflow of recently-formed oxygenated Okhotsk Sea Inter- North Pacific. The paleoreconstructions of the Okhotsk Sea environ- mediate Water (OSIW) and the inflow of old oxygen-depleted Deep Pa- ments help to understand global climate change. At present, the ex- cific Water (DPW) from the North Pacific(Fig. 1). tremely high primary production of the Okhotsk Sea is favored by the Recent studies provide evidence for significant glacial to interglacial variations of the Okhotsk Sea seasonal ice cover, terrestrial organic mat- fl ⁎ Corresponding author. ter in ow, marine productivity and circulation (Gorbarenko et al., E-mail address: [email protected] (N. Bubenshchikova). 2002a; Gorbarenko et al., 2010; Iwasaki et al., 2012; Nürnberg and

Fig. 1. A) Bathymetric chart of the Okhotsk Sea and location of core MD01-2415 (red star). Reference hydrological stations (open circle) and cores (black circles) are shown (for detailed information see Table S4 and Figures S1 and S2 of the Appendix). Solid line indicates mean sea ice extent in March during 1971–1991 (Rostov et al., 2001). B) Inset in the upper left corner shows general surface circulation pattern in the Okhotsk Sea and northwest Pacific. C) Inset in the low right corner shows dissolved oxygen (ml l−1) within the oxygen minimum zone at ~750–1500 m water depths (Bruyevich et al., 1960). The formation process of Okhotsk Sea Intermediate Water (OSIW) by: i) mixing of Dense Shelf Water (DSW) and North Pacific Water (NPW), ii) mixing driven by the inflowing dense Soya Water (SW), and iii) tidal mixing around the Kurile Straits (Freeland et al., 1998; Gladyshev et al., 2003; Moroshkin, 1966; Talley, 1991) is indicated. DPW = Deep Pacific Water, NPIW = North Pacific Intermediate Water. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Tiedemann, 2004; Seki et al., 2004; Seki et al., 2009; Seki et al., 2012). In MIS 11, and particularly MIS 11c, is considered a potential analog for particular, benthic foraminifera-based studies suggest increased pro- the present interglacial (MIS 1 or Holocene) and future climate, because ductivity, enhanced inflow of Old Pacific Water (DPW here) and weak- of the similarity in the orbital configuration (Loutre and Berger, 2003). ened outflow of the OSIW during interglacial marine isotope stage The global sea level rise during MIS 12/11 transition is seen as the youn- (MIS) 1, 5e and 9 compared to glacial MIS 2–5d, 6–8 and 10 (Barash gest analog for the sea level rise during MIS 2/1 transition, although dur- et al., 2001, Barash et al., 2006; Khusid et al., 2005). In high-resolution ing MIS 11c, the sea level stood 13 to 20 m above the present level sediment cores throughout the Okhotsk Sea, benthic foraminiferal as- (Fig. 2c) (Miller et al., 2005). To date, detailed variations of environmen- semblages dominated by bolivinids were found during termination tal conditions during MIS 11c in the Subarctic Pacific and surrounding (T) I, pointing to increased productivity, reduced oxygenation of bottom landmass are insufficiently studied because of a lack of appropriate sed- waters and intensification of the OMZ (Bubenshchikova et al., 2010; iment material. Recent pollen-based reconstructions for Lake Baikal and Gorbarenko et al., 2002a, Gorbarenko et al., 2010). Geochemical and Lake Elgygytgyn sediments indicate warmer and wetter climate condi- mineralogical studies assume the existence of anoxic bottom water con- tions over Siberia during MIS 11c as compared to MIS 1 (Melles et al., ditions in the Derugin Basin during TI (Derkachev et al., 2007)and 2012; Prokopenko et al., 2010). In the Okhotsk Sea, the available sedi- deglacial–early interglacial intervals of MIS 1, 5, 9 and 11 (Liu et al., mentological and geochemical data (including those for core MD01- 2006). To summarize, previous results suggest a basin-wide intensifica- 2415 under study) show that TV and TI and subsequent interglacials tion of the OMZ at low intermediate depths in the Okhotsk Sea during TI. of MIS 11c and MIS 1 were characterized by high productivity (Fig. 2a) Variations of the OMZ intensity during previous deglacial to interglacial (Iwasaki et al., 2012; Nürnberg and Tiedemann, 2004) and anoxic bot- intervals, however, remain largely unknown. tom water conditions in the Derugin Basin (Liu et al., 2006). Here, we 54 N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69

Fig. 2. a) The 46.23 m-long sedimentary color b* record of core MD01-2415 covering the last 1.1 million years (Nürnberg and Tiedemann, 2004), b) LR04 δ18Orecord(Lisiecki and Raymo, 2005), c) global sea level curve (Miller et al., 2005). Stratigraphical framework of core MD01-2415 (Nürnberg and Tiedemann, 2004) and the intervals under study are indicated. MIS = Marine Isotope Stage. present the first high-resolution study for the Okhotsk Sea focusing on intensity in the Okhotsk Sea during the latest MIS 12–11c and latest similarity and differences between the latest MIS 12–11c and latest MIS 2–1 and to discuss climate forcing of the regional environmental MIS 2–1 intervals, including potentially analogous interglacials over changes. late Quaternary time. This study applies mainly benthic foraminiferal assemblage compo- 2. Core location and modern oceanographic setting sition, which is a useful tool for reconstructing productivity and OMZ intensity in the past (Kaiho, 1994; Cannariato and Kennett, 1999; McKay The CALYPSO giant piston core MD01-2415 was recovered from the et al., 2005; Shibahara et al., 2007; Bubenshchikova et al., 2010;review northern slope of the Okhotsk Sea at 53°57.09′N, 149°57.52′E at 822 m in Jorissen et al., 2007). This approach relies on the concept of the pri- water depth (Fig. 1A) in the framework of the IMAGES program during mary control of organic matter flux to the sea floor and benthic oxygen- the WEPAMA 2001 cruise of the R/V Marion Dufresne (Holbourn et al., ation on modern deep-sea benthic foraminifera (review in Gooday, 2002). Below, we describe some key features of hydrology of the 2003). The organic matter flux (its quality, quantity and seasonality) is Okhotsk Sea and the core MD01-2415 location (for hydrological param- considered as the leading factor, while the oxygen strongly affects the eters at reference stations see Figures S1 and S2 of the Appendix). Win- − benthic fauna only at low concentrations below 1 or 0.5 ml l 1 ter sea ice extending as far south as 43°N is a major characteristic of the (Gooday, 2003; Jorissen et al., 2007). Early studies described the depen- Okhotsk Sea (Fig. 1A). Close to the location of core MD01-2415, the min- dence of the recent Okhotsk Sea benthic foraminifera on bathymetry imal, mean and maximal duration of the seasonal sea ice cover is esti- and water masses of the Okhotsk Sea or North Pacific origin (Inoue, mated as 0, 3, and 5 months per year, respectively (Fig. S1) (Rostov 1989; Saidova, 1961, 1997). Recently, live faunal observations showed et al., 2001). Relatively warm North PacificWater(NPW)flows into the correlation of the benthic foraminiferal assemblages with the organ- the southeastern Okhotsk Sea at different water depths (Fig. 1A) ic matter flux, duration of the seasonal ice cover, bottom water oxygen- preventing expansion of the seasonal sea ice cover and influencing pri- ation and calcite saturation (Bubenshchikova et al., 2008). Until now, mary productivity. Close to the studied site, annual primary production the ecology of the Okhotsk Sea benthic foraminifera remains insuffi- varies from 100 to 300 g C m−2 yr−1 (Arzhanova et al., 2002). These ciently studied. Before paleoreconstructions, we reviewed the existing high values might be due to nutrients derived from seasonal sea ice notion about the preference of selected benthic foraminifera for water melting, fluvial discharge and cyclonic eddy-induced upwelling near masses of the Okhotsk Sea and bottom water oxygen conditions. the Kashevarov Bank. The Amur River, northern coast rivers and Kam- The 46.23 m long core MD01-2415 from the Okhotsk Sea covers the chatka rivers add up to an averaged annual discharge of 371, 82.1 and last 1.1 million years (Fig. 2a) (Nürnberg and Tiedemann, 2004). This 52.3 km3 (63.3, 14 and 9% of total river discharge), respectively study focusses on two distinct core sections, from 23.49 to 22.01 m (Bezrukov, 1960). Plankton blooms commonly occur twice a year in and from 3.31 to 0 m core depth, representing the latest MIS 12–11c large areas of the Okhotsk Sea and close to the studied location (435 to 395 kyr BP) and latest MIS 2–1 (18 to 0 kyr BP), respectively (Fig. S1). Following sea ice retreat, siliceous microplankton (mainly dia- (Fig. 2a, b). We present an improved age model and high-resolution toms) dominates the main late spring/early summer plankton bloom quantitative data on the benthic assemblage composition, abundance because of the high silicate availability in the mixed layer (Broerse of the benthic and planktonic foraminifera and ice-rafted debris (IRD). et al., 2000). The secondary autumn maximum of coccolithophorids We apply the IRD and sediment magnetic susceptibility data to recon- and planktonic foraminifera coincides with the onset of sea surface struct the sea ice cover dynamics. To assess comprehensive variations cooling and a drop of the silicate content in the mixed layer (Broerse of productivity, we detail existing geochemical data on the biogenic et al., 2000). opal, total organic carbon, calcium carbonate and C/N ratio (Nürnberg In this study, we refer to several water masses. Dense Shelf Water and Tiedemann, 2004). We provide new evidence concerning ventila- (DSW) is a cold (0 b −1.7 °C) oxygen-rich shelf water mass tion changes using benthic foraminiferal δ13C data and 3 new and 21 (Shcherbina et al., 2004). The DSW, which is produced by brine rejec- formerly published AMS 14C-derived ventilation ages. The benthic as- tion during winter sea ice formation, is the main source of Okhotsk semblages are compared with the records reflecting changes in the Sea Intermediate Water (OSIW) (for details of the OSIW formation see sea ice cover, productivity and ventilation, factors which are known to Fig. 1C). The OSIW (~200 to 600–800 m) is characterized by an increase influence the benthic fauna and intensity of the OMZ. The main aims of temperature from 0.1 to 2 °C and decrease of oxygen content from 6 of our study are to investigate variations in the productivity and OMZ to 7 to 1.5 ml l−1 with depth (Fig. S2) (Freeland et al., 1998; Gladyshev N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69 55 et al., 2003; Moroshkin, 1966). Deep Pacific Water (DPW) (~600–800 to or 6 samples. We focus on the distribution of the relative abundance 1500 m) appears as a relatively warm and oxygen-depleted layer of 13 dominant species (with N15% at least in one sample) (for the with up to 2–2.45 °C and less than 1.5 ml l−1 oxygen content (Fig. S2) species absolute abundances see Fig. S4). The absolute abundance (Freeland et al., 1998; Moroshkin, 1966). The DPW originates from (number) of total planktonic, total benthic and dominant benthic waters of the mesothermal layer (up to 3–3.5 °C 3.5 °C at depths foraminiferal taxa is expressed as the number of specimens per gram 200–1000 m) and deeper layers (up to 2–3 °C at depths dry sediment (PFN, BFN, #FN, sp. g−1, respectively). On the basis of 1000–2000 m) in the western periphery of the Subarctic Gyre the bathymetrical distributions of the recent benthic foraminifera by (Bogdanov and Moroz, 2004). The source of the mesothermal layer in Saidova (1961) and Inoue (1989), we related the dominant species the Subarctic Gyre is warm water of the Kuroshio Current transported with the Okhotsk Sea water masses, having distinct oxygen concentra- via the Subarctic Current, Alaskan Stream, Kamchatka and Oyashio Cur- tions (described in Section 2). Also, we linked the dominant calcareous rents (Fig. 1B) (Rogachev et al., 2007). Kurile Basin Water (KBW) is species to the dysoxic (0.1–0.3 ml l−1), suboxic (0.3–1.5 ml l−1), low found between 1500 and 3374 m water depths in the southern part of oxic (1.5–3mll−1) and high oxic (3–6mll−1) bottom water indicator the Okhotsk Sea and is marked by a decrease of temperature from 2.2 groups following Kaiho (1994) and the species water mass preferences. to 1.9 °C and an increase of oxygen content from 1.5 to 2.3 ml l−1 The majority of the species, except for Bolivina spissa, Brizalina with depth (Moroshkin, 1966). The Okhotsk Sea OMZ, with oxygen con- subspinescens and Pullenia bulloides, covers a wide range of the bottom centrations from 0.5 to 1.2–1.5 ml l−1, occurs between ~750 and water oxygen conditions (several indicator groups) (Table 5). Benthic 1500 m water depths (Figs. 1candS2)(Bruyevich et al., 1960). Lowest assemblage preservation was estimated as good, moderate and bad by bottom water oxygen concentrations of 0.3 ml l−1 are observed at using dissolution indices: 1) the ratio of benthic to planktonic foraminif- ~1700 m water depth in the Derugin Basin (Pavlova et al., 2008). Core era: B/P ratio% = 100 ∗ BF/(BF + PF), 2) the ratio of “agglutinated” MD01-2415 at 822 m water depth is located near the upper edge of to calcareous benthic foraminifera: Agl/Cal ratio% = 100 ∗ Agl/ the DPW and OMZ, whose boundaries coincide here. The Okhotsk Sea (Agl + Cal), and 3) the ratio of fragmented to whole tests: Frag waters are undersaturated with respect to CaCO3 down to 200–500 m ratio% = 100 ∗ Frag/(Frag + whole). The benthic assemblages were water depths (saturation degree of calcite, Lc ≤ 1) and highly undersat- discriminated with Q-mode factor analysis (Klovan and Imbrie, 1971). urated within the OMZ (Lc = 0.7) and down to 1500 m water depth in Detailed information about the generation of the foraminiferal data is the Derugin Basin (Lc = 0.64–0.70) and Kurile Basin (Lc = 0.7–0.8) given in Text SI of the Appendix.

(Kaiser, 2000; Lyakhin, 1970; Pavlova et al., 2008). The CaCO3 content in the Okhotsk Sea surface sediments is commonly low (b1wt.%)at 3.2. Color b*, magnetic susceptibility, IRD and occurrence of cryptotephra sites shallower than 500 m and deeper than 1500 m water depth, layers while it increases up to 5–10 wt.% in the depth range of 1000 to 1200 m at bathymetric highs in the central Okhotsk Sea (Bezrukov, The sediment color b* and magnetic susceptibility (MS) were mea-

1960; Kaiser, 2000). The overall low CaCO3 concentrations in surface sured every 2 and 4 cm reflecting mean temporal resolution of 100 sediments is caused by both a limited CaCO3 production in the surface and 2000 years, respectively (Table 1)(Nürnberg and Tiedemann, waters and an inflow of carbonate-corrosive deep waters from the 2004). We use the color b* as an indicator of the Okhotsk Sea surface North Pacific. The Carbonate Compensation Depth (CCD) is at 1500 m productivity, mainly of biogenic opal production (Nürnberg and water depth, defined by less than 1% CaCO3 in sediments of the Derugin Tiedemann, 2004). The MS, which reflects the content of magnetizable and Kurile Basins. minerals in the sediment, is applied as a proxy of terrigenous supply, mainly resulting from the sea ice cover dynamics (Gorbarenko et al., 3. Material and methods 2002b). Lithic and volcanic grains were counted in an aliquot (with more than 200 grains) of our foraminiferal samples. Their abundance 3.1. Foraminiferal data was expressed as the number of grains per gram dry sediment (grains g−1). The abundance of the lithic grains is presented as ice- For foraminiferal analysis, 1 cm thick sediment samples were taken rafted debris (IRD, grains g−1). Visible tephra layers were not found in every 1 cm for the 23.49–22.01 m interval and every 5 cm for the the studied intervals of core MD01-2415 (Derkachev et al., 2012). Two 3.31–0 m interval. This potentially provides ~300 years mean temporal layers with increased abundances of volcanic glass at 2313–2303 and resolution for the foraminiferal data (Table 1). For foraminiferal counts, 216–202 cm were defined as cryptotephras (Figs. 3aand7b). The the size fraction N125 μm was used to keep the data comparable to the younger cryptotephra was referred to the tephra KO associated with previous results (Bubenshchikova et al., 2008, 2010; Khusid et al., the Kurile Lake caldera on southern Kamchatka, which has been 2005). In rich samples, at least 300 planktonic and 300 benthic dated 8.46 kyr BP (Zaretskaia et al., 2001). The correlation of the specimens were counted in an aliquot of sediment sample. In total, older cryptotephra to Kamchatkan source volcanoes is unclear 109 benthic taxa were identified following mainly the taxonomy by (V. Ponomareva personal communication 2014). Saidova (1961), Fursenko et al. (1979) and Inoue (1989) (for a full reference list see Table S1). The number of counted specimens varies from 3.3. Geochemical data 1 to 615 during latest MIS 12–11c and from 43 to 585 during latest MIS 2–1 (Fig. S3). Species percentages were only calculated for samples with Biogenic opal (wt.%) was measured using the automated leaching N80 specimens. Forty three samples with b80 specimens, covering method of Müller and Schneider (1993) at GEOMAR. Previous 2274–2271, 2264–2259 and 2233–2201 cm core depth (Fig. S3), were (Nürnberg and Tiedemann, 2004) and new biogenic opal concentra- used to calculate averaged species percentages as a mean value over 5 tions are available every 10 cm in both studied intervals, except

Table 1 Information about sampling resolution of paleorecords for the studied intervals in core MD01-2415.

Number of samples/mean temporal resolution, years Marine isotope Core depth, m Age, kyr BP IRD, Magnetic Biogenic TOC, CaCO , C/N Uvigerina spp. Uvigerina spp. stage Foraminifera Color b* 3 V.glasses susceptibility opal Ratio δ18O δ13C

0–3.31 0–18 Latest MIS 2-1 67/300 67/300 168/100 82/200 20/900 54/400 74/250 40/450 22.01–23.49 395–435 Latest MIS 12-11c 149/300 or 2000 149/300 74/500 38/1000 16/2500 31/1300 29/− 18/− 56 N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69

Fig. 3. Stratigraphical framework for the latest MIS 12–11c and latest MIS 2–1 of core MD01-2415 showing records of: a) Uvigerina spp. δ18O (this study; Nürnberg and Tiedemann, 2004), b) sedimentary color b* record, c) NGRIP and LR04 δ18O reference records (Lisiecki and Raymo, 2005; NGRIP members, 2004; Rasmussen et al., 2006), d) linear sedimentation rates (LSR, cm kyr−1), e) mass accumulation rates of sediments (MAR, g cm−2 kyr−1). Arrows show the calibrated AMS 14C dates in cal. years BP (see Tables 2 and 3). Stars mark two excluded samples with the exceptionally high δ18O values during the MIS 11c. Boxes indicate location of two cryptotephras referred to the 8.46 kyr BP tephra KO and an undated tephra. The last 2.4 kyr LSR and MAR are accepted as overestimations due to coring disturbance (Nürnberg and Tiedemann, 2004). See Fig. 6 and text for further explanations. for the latest MIS 1, which has been analyzed every 20–50 cm. Measure- organic matter. Previous (Nürnberg and Tiedemann, 2004) and new ments of total carbon (TC, wt.%), total organic carbon (TOC, wt.%) and TC, TOC, TN, CaCO3 (wt.%) and C/N ratios are available every 5 cm for total nitrogen (TN, wt.%) were performed with a Vario EL III elemental the latest MIS 12–11c and every 2–10 cm for the latest MIS 2–1. Mean analyzer at the Otto Schmidt Laboratory, St.-Petersburg, Russia. TC temporal resolution of the geochemical data varies from 400 to (wt.%) was determined from bulk sediment samples. TOC (wt.%) was 2500 years (Table 1). measured on de-calcified sediments. CaCO3 (wt.%) was calculated as the difference between TC and TOC (wt.%) multiplied by 8.333. C/N ra- 3.4. AMS 14C dating and stable isotope data tios were evaluated by using previous (Nürnberg and Tiedemann, 2004) and new TOC and TN data. First, from the TN (wt.%) for the latest Three coeval benthic–planktonic and one benthic accelerator mass MIS 12–11c and latest MIS 2–1 we subtracted the intercept values of spectrometry radiocarbon datings (AMS 14C) were obtained for core 0.0146 and 0.004 (at zero TOC wt.%), respectively, to correct the data MD01-2415, performed at the Leibniz-Laboratory for Radiometric for terrigenous nitrogen content following Goni et al. (1998).Then, Dating and Stable Isotope Research, Kiel (Table 2). Measurements the atomic TOC versus TN (C/N) ratios were calculated. The С/N ratio, were made on monospecific N. pachyderma sin. tests from the which varies between 5 and 8 for marine phytoplankton and between 125–250 μm fraction and on mixed benthic foraminiferal tests from 25 and 35 for terrestrial plants (Emerson and Hedges, 1988; Redfield the 125–500 μm fraction. Three planktonic AMS 14C dates were convert- et al., 1963), is used as a proxy of marine and terrestrial sources of ed into calibrated 1-sigma calendar ages with the Calib 6.0 program

Table 2 Accelerator mass spectrometry radiocarbon dates, B-P age differences and calibrated calendar ages in core MD01-2415.

Laboratory Depth, 14C AMS age planktonica, 14C AMS age benthicb, Benthic–planktonic (B–P) age Reservoir age Calendar age 1σ, Calendar age, number cm years B.P. years B.P. difference, years correction, years years B.P. years B.P.

KIA 45315 76a 3235 ± 25 900 2358–2471 2415 KIA 42508 76b 5190 ± 45 1955 ± 50 KIA 42510 236b 10,900 ± 60 1925 10,240–10,395 10,318 KIA 45316 242a 10,295 ± 50 900 10,573–10,707 10,640 KIA 42511 242b 11,320 ± 70 1025 ± 90 KIA 45317 272a 12,050 ± 55 900 12,945–13,119 13,032 KIA 42512 272b 13,210 ± 80 1160 ± 100

a Neogloboquadrina pachyderma sinistral tests from N125–250 μm. b Mixed benthic foraminiferal tests from N125–500 μm. N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69 57

(Reimer et al., 2009) after correction for the Okhotsk Sea surface water interval due to under pressure and piston suction during core retrieval reservoir age of 900 years (Southon et al., 2005). One benthic date for (Skinner and McCave, 2003). We rely mainly on distribution patterns the 236 cm sample was calibrated using a bottom water reservoir age of accumulation rates of TOC (TOC AR, g cm−2 kyr−1), benthic forami- correction of 1925 years. This value was obtained as the difference be- nifera (BFAR, sp. cm−2 kyr−1) and planktonic foraminifera (PFAR, tween raw AMS 14C benthic and planktonic (B–P) ages of 1025 years sp. cm−2 kyr−1), which were calculated as TOC (wt.%), BFN (sp. g−1) for the nearest 242 cm sample (Table 2) plus the surface water reservoir and PFN (sp. g−1) multiplied by MAR (g cm−2 kyr−1), respectively. age of 900 years. We use 3 own and 21 published AMS 14C-derived B–P For the last 2.4 kyr, the LSR, MAR, BFAR, PFAR and TOC AR (Figs. 3d–e, age differences for 8 the Okhotsk Sea cores (Tables 2 and S2) to recon- 4a–band7e) are considered as overestimated due to coring disturbance struct ventilation ages of low intermediate water (~700 to 1800 m (Nürnberg and Tiedemann, 2004). water depths) during the latest MIS 2–1. The B–P age difference of ~900 years for the top sample in core LV28-4-4 at 674 m water depth 4. Results (Kozdon, 2002) is used as a modern reference value. Measurements of δ18Oandδ13Cweremadeon2–3 specimens of 4.1. Comparison of the climatic events Uvigerina akitaensis and Uvigerina peregrina var. dirupta from the 250–500 and 125–500 μm size fractions. Samples were analyzed with In the following, we compare our proxy records mainly in terms of a Finnigan MAT 253 mass spectrometer with an automated Kiel carbon- i) latest glacials: MIS 12 (435 to 429 kyr BP) and MIS 2 (18 to 14.7 kyr ate preparation line at GEOMAR. The Uvigerina spp. δ18Oandδ13Cdata BP); ii) early deglacial stages: TVa (429 to 422 kyr BP) and TIa (14.7 to are available every 1–20 cm for the latest MIS 2–1. Mean temporal res- 12.8 kyr BP); iii) late deglacial stages: TVb (422 to 416 kyr BP) and TIb olution of the Uvigerina spp. δ18Oandδ13C records for this period is 250 (11.7 to 8.5 kyr BP); iv) full interglacials: early MIS 11c (416 to and 450 years, respectively (Table 1). Uvigerina specimens were very 405 kyr BP) and MIS 1 (8.5 to 0 kyr BP) and v) late MIS 11c (405 to rare or absent at 2349–2341, 2274–2250 and 2233–2201 cm core 395 kyr BP). Figs. 4–8 and S4 show the Prebølling, Oldest Dryas, Bølling, depth (Fig. 3a) that resulted in scattered isotope data during the latest Older Dryas, Allerød, Younger Dryas, Preboreal and Boreal periods dur- MIS 12–11c and their limited use in stratigraphy and paleoceanographic ing TI, consistent with the Greenland ice core chronology (Rasmussen interpretation. Our new data do not support previously published heavy et al., 2006). Possible analogous events during TV are defined based on δ18O values for the 2230 cm and 2280 cm samples (Nürnberg and the similarity in the abundance maxima of U. akitaensis, Takayanagya Tiedemann, 2004) interpreted here as reworking (Fig. 3a). The delicata and B. spissa between TV and TI (for a detailed description see Uvigerina spp. δ13C data, which are indicative of ambient seawater Fig. 5). The TV events are also presented in Figs. 4, 6–8 and S4. δ13C nutrient and oxygen-levels, are used as a proxy of water mass changes. 4.2. Benthic assemblages: preservation, foraminiferal numbers and accumulation rates 3.5. Core stratigraphy and accumulation rate calculations Low or zero BFN at 435–429, 416–414.5, 413.2–411.5, ~407.8, The age model of the latest MIS 2–1(3.31–0 m core depth) relies on 405–395, ~15.7 and ~14.8 kyr BP (marked by the hatched bands in four calibrated AMS 14C dates and six climatic events of the NGRIP δ18O Figs. 4 and 5–6), coinciding with intervals with b80 counted specimens record (NGRIP members, 2004; Rasmussen et al., 2006) recognized in (Fig. S3), may reflect reduced productivity, bad preservation or both. the Uvigerina spp. δ18O or color b* curves (Table 3 and Fig. 3a–c). The Bad benthic preservation is assumed at 435–429 kyr BP because of identification of the 8.2 kyr BP cooling event at 196 cm core depth is zero BFNs coupled with maximal IRD abundances (Figs. 4aand7b) in agreement with the underlying cryprotephra correlated to the and at 416–414.5, 413.2–411.5 and 405–395 kyr BP because low BFNs 8.46 kyr BP tephra KO (Fig. 3a). The stratigraphy of the latest MIS co-occurred with high Agl/Cal ratios (Fig. 4a, d). During TVa, moderate 12–11c (23.49–22.01 m core depth) is based on ten age control points benthic preservation is indicated by the increased B/P ratio (N35%) obtained via the correlation of the color b* or Uvigerina spp. δ18O curves and Frag ratio (N35% mainly due to fragmented tests of U. akitaensis) (Table 3 and Fig. 3a–c) to the LR04 δ18O record (Lisiecki and Raymo, (Fig. 4c, e). For the remaining TVb–early MIS 11c and the latest MIS 2005). The resulting mass accumulation rates of sediments (MAR, 2–1, we observed good preservation of the benthic assemblages, as indi- gcm−2 kyr−1) were calculated from linear sedimentation rates (LSR, cated by low dissolution indices (Fig. 4c–e). BFNs and BFAR reach cm kyr−1) between the new age control points (Table 3) multiplied highest values, up to 235 sp. g−1 and 1250 sp. cm−2 kyr−1,respectively, by the dry bulk densities (DBD, g cm−2)(Nürnberg and Tiedemann, during the latest MIS 12–11c and up to 124 sp. g−1 and 4409 sp. cm−2- 2004). LSR vary between 1.7 and 7 cm kyr−1 during the latest MIS kyr−1, respectively, during the latest MIS 2–1(Fig. 4a). Both proxies 12–11c and between 7.3 and 47.8 cm kyr−1 during the latest MIS 2–1 show maxima during TV (427 to 419 kyr BP) and TI (Allerød and (Fig. 3d). MAR range from 1.1 to 5.6 g cm−2 kyr−1 during the latest Preboreal). PFNs and PFAR attain maximal values, up to 2228 sp. g−1 MIS 12–11c and from 5.8 to 37.3 g cm−2 kyr−1 during the latest MIS and 6250 sp. m−2 kyr−1, respectively, in the older interval and up to 2–1(Fig. 3e). The LSR and MAR may be overestimated for the younger 1959 sp. g−1 and 45,421 sp. cm−2 kyr−1, respectively, in the younger

Table 3 Age control points, linear sedimentation rates and relevant events in core MD01-2415.

Depth, cm Age, cal. kyr BP Event during the latest MIS 2-1 LSR, cm kyr−1 Depth, cm Age, cal. kyr BP Event during the latest MIS 12-11c LSR, cm kyr−1

0 0 Core top 14 18 76 2.42 Calibrated C AMS age on PF 31.4 2202 395 Color b*max-LR04 δ Omin 18 18 18 196 8.20 Cooling 8.2 ka, δ Omax-NGRIP δ Omin 20.8 2212 397 Color b*max-LR04 δ Omin 5.0 14 18 236 10.32 Calibrated C AMS age on BF 18.9 2234 405 MIS 11.3, color b*max-LR04 δ Omin 2.8 14 18 242 10.64 Calibrated C AMS age on PF 18.8 2252 410 Color b*max-LR04 δ Omin 3.6 18 18 253 11.70 Younger Dryas, color b*min-NGRIP δ Omin 10.4 2272 415 Color b*max-LR04 δ Omin 4.0 18 18 261 12.80 Younger Dryas, color b*min-NGRIP δ Omin 7.3 2288 420 Color b*max-LR04 δ Omin 3.2 14 18 18 272 13.03 Calibrated C AMS age on PF 47.8 2316 424 MIS 12.0, δ Omin-LR04 δ Omin 7.0 18 18 18 286 14.08 End of Bølling, color b*min-NGRIP δ Omin 13.3 2330 426 δ Omax-LR04 δ Omax 7.0 18 18 18 18 295 14.70 Onset of Bølling, δ Omax-NGRIP δ Omin 14.5 2336 427 δ Omin-LR04 δ Omin 6.0 18 18 18 330 17.90 MIS 2.22, δ Omax-NGRIP δ Omin 10.9 2348 434 MIS 12.2, color b*min-LR04 δ Omax 1.7 58 N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69

Fig. 4. Comparison of the latest MIS 12–11c and latest MIS 2–1 in the core MD01-2415 records of: a) benthic foraminiferal number (BFN, sp. g−1)andaccumulationrates(BFAR, sp. cm−2 kyr−1), b) planktonic foraminiferal number (PFN, sp. g−1) and accumulation rates (PFAR, sp. cm−2 kyr−1), c) B/P ratio (%), d) Agl/Cal ratio (%), e) Frag ratio (%). The last 2.4 kyr BFAR and PFAR are accepted as overestimations due to coring disturbance (Nürnberg and Tiedemann, 2004). See Fig. 6 and text for further explanations. one (Fig. 4b). Both proxies have maxima during early MIS 11c (411.5 to maxima at 416–414.5, 413.2–411.5 and 405–395 kyr BP (Fig. 6b, e). 405 kyr BP), TIb (Preboreal) and MIS 1 (3.2 to 1.2 kyr BP). The A. angulosa assemblage dominates during the latest MIS 2 (Fig. 6e).

4.4. Distribution of sedimentological parameters 4.3. Dominant benthic foraminiferal species and Q-mode assemblages Highest IRD abundances occur during the latest MIS 12 (Fig. 7b). In- Nine benthic foraminiferal species are dominant in each studied increased IRD and/or MS values characterize TVa, the late MIS 11c and lat- terval, while five species are common (Fig. 5). TVa and TIa are character- est MIS 2–TI (Fig. 7a, b). During MIS 1, both proxies drop to lower values ized by maximal percentages of U. akitaensis (up to 85 and 45%, compared to TVb–early MIS 11c. The biogenic opal content increases respectively) and T. delicata (up to 60 and 15%, respectively) (Fig. 5a, more during TVb (up to 30%) compared to TIb (up to 20%). The biogenic b). TVb and TIb are marked by a maximal abundance of B. spissa (up opal contents and color b* values reach highest values during MIS 1 (up to 40 and 25%, respectively) (Fig. 5c). Valvulineria sadonica reaches max- to 58% and 17.3, respectively), exceeding those of MIS 11c (up to 39% imal percentages during TVa (up to 30%) and TIb (~Boreal up to 26%) and 9.2, respectively). The CaCO3 content does not exceed 1.1% during (Fig. 5d). B. subspinescens, which only has an elevated abundance in TV (426 to 419 kyr BP) and attains maxima up to 3.5, 4.6 and 7% during the younger interval, shows a maximum during TIb (~Boreal up to TI: Allerød, Preboreal and Boreal, respectively (Fig. 7d). The CaCO3 25%) (Fig. 5g). Islandiella norcrossi appears since TVb and after TI and at- content shows higher values during MIS 1 (up to 8.4%) compared to tains maximal abundances (up to 55%) during both full interglacials early MIS 11c (up to 3.8%). Highest TOC percentages, up to 1.49 and (Fig. 5e). Pullenia bulloides has maximal percentages (up to 18%) during 1.25%, are found during TV (425 to 419 kyr BP) and TIb (Preboreal), early MIS 11c, while Oridorsalis umbonatus shows highest percentages respectively. Highest TOC AR, up to 0.08 and 0.31 g cm−2 kyr−1, are cal- (up to 22%) during MIS 1 (Fig. 5f). Late MIS 11c is marked by high quan- culated for TV and TIa (Allerød), respectively. The C/N ratio varies from tities of the taxa with “agglutinated” tests: Miliammina herzensteini, 8.1 to 15.6 during the latest MIS 12–11c and from 7.3 to 14.1 during the Karreriella baccata and Martinottiella communis (up to 55, 75 and 40%, latest MIS 2–1(Fig. 7f). The highest C/N ratios are observed during both respectively) (Fig. 5g–i). Maximal percentages of Angulogerina angulosa deglaciations, in particularly during TV. (up to 25%) and Uvigerina auberiana (up to 35%) occur during the latest MIS 2 (Fig. 5h, i). 4.5. Uvigerina spp. δ13C and ventilation age data The Q-mode factor analysis recognized four and five assemblages explaining 93.4 and 95.9% of the total faunal variability in the The Uvigerina spp. δ13C varies from −1.04 to −1.76‰ during the younger and older intervals, respectively (Table 4). Highest scores of latest MIS 12–11c and from −0.18 to −2.05‰ during the latest MIS U. akitaensis, B. spissa and I. norcrossi define three assemblages (factors 2–1(Fig. 8a). Highest Uvigerina spp. δ13C values are observed at 1 to 3) in both intervals. During the latest MIS 2–1, the A. angulosa as- 18–15.8 kyr BP, while lowest ones are found during TIa and TIb, at semblage (factor 4) is characterized by maximal scores of A. angulosa ~432.3, 425.4, 422.6 and 2.2 kyr BP. During the latest MIS 2–1, the ven- and U. auberiana. During the latest MIS 12–11c, the “agglutinated” as- tilation ages of low intermediate water (Fig. 8b) vary in the range from semblage (factor 4) is defined by maximal scores of M. herzensteini, 160 to 1955 years (Tables 2 and S2). Minimal ventilation ages are found K. baccata and M. communis, while the T. delicata assemblage (factor at 17.2–14.7 kyr BP (except for an old age at 17.2 kyr BP in the deepest 5) is characterized by maximal score of T. delicata. There is a similarity core LV29-114-3) (Table S2 and Figs. 1, 8b). During TIa and TIb, the in the succession of the three assemblages in both intervals. The ventilation ages increase compared to 17.2–14.7 kyr BP, while they fluc- U. akitaensis assemblage dominates during TVa and TIa, while the tuate highly during MIS 1. B. spissa assemblage shows a maximum during TVb and TIb (Fig. 6a, c). The I. norcrossi assemblage appears since TVb and after TI and dom- 5. Discussion inates during both interglacials: at 414.5–413.2, 411.5–405 and 8.5–0 kyr BP (Fig. 6d). The T. delicata assemblage has a maximum at In the following, we first present an interpretation of the benthic as- 425.2–423.8 kyr BP, while the “agglutinated” assemblage shows semblages defined by the Q-mode factor analysis (Table 4). This relies N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69 59

Fig. 5. Comparison of the latest MIS 12–11c and latest MIS 2–1 in the core MD01-2415 records of: a–i) relative abundance of the dominant benthic foraminifera (%). Hatched bands mark intervals with low abundant benthic assemblages (0–80 counted specimens). Dashed lines show the averaged species percentages. Green bands during the TI indicate climatic events identified following the Greenland ice core chronology (NGRIP members, 2004; Rasmussen et al., 2006) and recognized in the species percentages maxima as follows: Prebølling at 15.6–14.9 kyr BP (U. akitaensis), Bølling 14.7–14.1 kyr BP (U. akitaensis), Allerød 14–12.8 kyr BP (T. delicata), Preboreal 11.7–10.3 kyr BP (B. spissa)andBoreal10.3–8.5 kyr BP (B. subspinescens). Green bands during the TV indicate climatic events identified on the basis of the similarity in the species percentage maxima between the TI and TV as follows: 428.8–426 kyr BP (U. akitaensis), 425.2–423.8 kyr BP (T. delicata), 422–419 and 418.2–416 kyr BP (B. spissa). MIS = Marine Isotope Stage. T = Termination. YD = Younger Dryas. See text for further explanations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) on the environmental preferences of the dominant species of core (Table 4), may reflect prolonged seasonal ice cover and a relatively MD01-2415 overviewed in Table 5 (for a detailed discussion see also low and pulsed flux of organic matter compared to other dominant spe- Text S2). The previously designated intervals of the moderate and bad cies (Table 5). As U. auberiana is typically found below the OMZ in the preservation of the benthic foraminifera (Section 4.2) are taken into modern Okhotsk Sea (Table 5), its co-occurrence with A. angulosa account when interpreting the Q-mode assemblages. Afterwards, we supports the disappearance of the OMZ. discuss the influence of various environmental factors on the productiv- The high-productivity U. akitaensis assemblage during TVa and TIa ity and intensity of the OMZ by using all our proxy records for the time and the low oxygen-tolerant B. spissa assemblage during TVb and TIb in- intervals defined by the downcore distribution of the Q-mode assem- dicate a decrease of bottom water concentrations (from suboxic to blages. Fig. 9 shows close similarity in environmental changes for the dysoxic ones) and a gradual intensification of the OMZ during both de- latest MIS 12–11c and latest MIS 2–1. glaciations (Figs. 6a, c and 9). We explain the maximum of the T. delicata assemblage at 425.2–423.8 kyr BP and the absence of an analogous 5.1. Benthic assemblages as indicators of paleoenvironments maximum of this assemblage during Ta (Fig. 6b) by higher productivity during TVa, evidenced also by higher BFNs and TOC contents compared During the latest MIS 2, the low-productivity A. angulosa assemblage to TIa (Figs. 4a and 7e). The high values of U. akitaensis, T. delicata and indicates suboxic to high oxic bottom waters and the absence of an OMZ B. spissa in the same name assemblages (Tables 4 and 5) indicate that (Figs. 6eand9). A. angulosa may denote a relatively low and suspended a high and sustained flux of particulate organic matter resulting from in- flux of organic matter caused by strong downslope currents, originating creased primary production caused the OMZ intensifications. At pres- from the vicinity of the glacial shelf edge and/or the area of enhanced ent, B. spissa is a rare species on the Okhotsk Sea continental slope water mass production (Table 5). Since in the modern Okhotsk Sea bathed in the DPW and is a key species of the Northwest Pacific OMZ A. angulosa is typical for shelf to continental slope environments bathed (Table 5). Thus, the high quantity of B. spissa in the same name assem- in well-oxygenated DSW and OSIW (Table 5), high quantities of this blages also suggests enhanced inflow of the DPW, leading to the OMZ species may point to the strengthened formation of these water masses. intensifications. As at present V. sadonica and B. subspinescens mostly in- U. auberiana, an important constituent of the A. angulosa assemblage habit deeper subsurface sediments, increased quantities of these species 60 N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69

Fig. 6. Comparison of the latest MIS 12–11c and latest MIS 2–1 in the core MD01-2415 records of: a–e) Factor loading of the Q-mode benthic assemblages or factors (F). Dashed lines represent the factor loading derived from the averaged species percentages. Reference curves are: f) global sea level (solid line after Waelbroeck et al., 2008; dashed line after Miller et al., 2005), g) June insolation at 60° N (Berger and Loutre, 1991). Light blue arrows indicate similar sea level rises during the TV and TI when main changes of the benthic assemblages occurred. See Table 4 for factor variances and scores of the dominant species. See Fig. 5 and text for further explanations. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 7. Comparison of the latest MIS 12–11c and latest MIS 2–1 in the core MD01-2415 records of: a) magnetic susceptibility (MS, SI 10−5), b) absolute abundance of the ice rafted debris −1 −1 (IRD, grains g ) and volcanic glasses (V.glasses, grains g ), c) biogenic opal (wt.%) and sedimentary color b*, d) calcium carbonate (CaCO3, wt.%), e) total organic carbon (TOC, wt.%) and accumulation rates (TOC AR, g cm−2 kyr−1), f) C/N ratio (c–e: this study; Nürnberg and Tiedemann, 2004). Two maxima of the V.glasses abundances represent cryptotephras referred to the 8.46 kyr BP tephra KO and an undated tephra. The last 2.4 kyr TOC AR are accepted as overestimations due to coring disturbance (Nürnberg and Tiedemann, 2004). See Fig. 5 and text for further explanations. N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69 61

Fig. 8. a) Comparison of the latest MIS 12–11c and latest MIS 2–1 in the core MD01-2415 records of Uvigerina spp. δ13C(‰ vs. PDB). b) Ventilation ages of low intermediate water (years) defined as the benthic–planktonic (B–P) AMS 14C age differences in core MD01-2415 (Table 2) and reference cores: V34-98 (Gorbarenko et al., 2002a), LV28-4-4 (Kozdon, 2002), 936 (Gorbarenko et al., 2004), LV28-40-5 (Gorbarenko et al., 2007), LV27-2-4 (Gorbarenko et al., 2010), LV29-114-3 and SO178-13-6 (Max et al., 2014). See Fig. 1 for the core locations and Table S2 for additional information. in the U. akitaensis, T. delicata and B. spissa assemblages (Tables 4 and 5) high (although less than deglacial) and pulsed flux of predominantly may indicate a high and sustained flux of degraded organic matter orig- particulate organic matter resulted in weakened oxygen consumption inating from primary production and refractory organic matter supplied and OMZ. During MIS 1, the I. norcrossi assemblage, with higher from the upper slope and shelf. Together with a number of the C/N ratio quantities of the phytodetritus-related O. umbonatus and the high- peaks during TV and TI (Allerød and Boreal) (Fig. 8f), this would imply productivity T. delicata, B. subspinescens and V. sadonica (Tables 4 and that the downslope supply of organic matter contributed to the OMZ in- 5), provides evidence for a somewhat higher flux of particulate and de- tensifications. During TVa, the U. akitaensis assemblage, with higher graded organic matter from primary production, more enhanced oxy- values of U. akitaensis and V. sadonica (Table 4), correlates with higher gen consumption and a less pronounced weakening of the OMZ. In TOC contents, BFNs and Frag ratios in comparison to TIa (Figs. 4a, c, e comparison to MIS 1, during early MIS 11c, the I. norcrossi assemblage, and 7e). This reflects both the high productivity and moderate benthic with higher values of overall less productive taxa with dissolution- preservation due to the decay of organic matter during TVa. During resistant tests, such as P. bulloides, M. herzensteini and K. baccata TVb, the B. spissa assemblage, with higher quantities of B. spissa and (Tables 4 and 5), points to a slightly reduced flux of particulate organic the lack of B. subspinescens, V. sadonica and U. akitaensis (Table 4), points matter, possibly a weak downslope supply of degraded and refractory to even higher productivity and a stronger (more oxygen-depleted) organic matter and more enhanced carbonate dissolution in sediments OMZascomparedtoTIb. located near the CCD. During both full interglacials, the phytodetritus-related I. norcrossi During MIS 11c, the “agglutinated” assemblage containing assemblage indicates suboxic to high oxic bottom waters and a weak- M. herzensteini, K. baccata and M. communis indicates decreased produc- ened OMZ (Figs. 6d and 9). The highest quantity of I. norcrossi in the tivity and lowered carbonate saturation of bottom and pore waters, same name assemblages (Tables 4 and 5) suggests that moderate to leading to strong carbonate dissolution in sediments and shoaling of

Table 4 The Q-mode factor analysis: variances and scores (N0.3) of the dominant benthic foraminiferal species in core MD01-2415.

Latest MIS 2–1 Latest MIS 12–11c

Variance Factor 2: the Factor 3: the Factor 1: the Factor 4: the Factor 1: the Factor 3: the Factor 2: the Factor 4: the Factor 5: the U. akitaensis B. spissa I. norcrossi A. angulosa U. akitaensis B. spissa I. norcrossi "agglutinated" T. delicata assemblage assemblage assemblage assemblage assemblage assemblage assemblage assemblage assemblage

Variance 18.7 16.8 48.2 9.7 39.1 11.1 28.3 9.7 7.7 Cumulative variance 35.5 83.7 93.4 50.2 78.5 88.2 95.9

Species Scores Uvigerina akitaensis 5.1 1.2 0.3 5.6 Takayanagia delicata 1.6 0.9 0.4 5.4 Bolivina spissa 3.0 5.6 Brizalina subspeniscens 3.4 1.7 –––– – Valvulineria sadonica 0.3 2.1 1.3 0.9 0.4 1.7 Islandiella norcrossi 0.6 4.0 1.2 5.5 Oridorsalis umbonatus 2.9 Pullenia bulloides –––– 0.9 Miliammina herzensteini 0.8 0.3 3.5 Karreriella baccata 0.4 4.4 Martinottiella communis –––– 1.0 Angulogerina angulosa 3.6 Uvigerina auberiana 3.5 62 N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69 the CCD relative to the remainder of MIS 11c (Tables 4–5 and Fig. 6e). both TV and TI, the global sea level rise has been estimated to have The “agglutinated” assemblage co-occurs with low or zero BFNs, PFNs the closest large magnitude at least over the last 1.1 million years and CaCO3 contents (Figs. 4a, b and 7d) and thus underlines the bad (Fig. 2c). We note that the changes in the benthic assemblages occurred benthic preservation state in these intervals (Section 4.2). at times of close standings of global sea level during both TV and TI (marked by the blue arrows in Fig. 6f). We propose that the orbitally 5.2. Disappearance of the OMZ during the latest MIS 12 and the latest MIS 2 forced similar sea level rises might in fact have been the main force of the close gradual OMZ intensifications, as it might have induced both During the latest MIS 12, surface productivity was suppressed by an a large offshore supply of organic matter and a high surface productivity almost perennial sea ice cover with periods of active ice rafting, as that resulted in a high and sustained flux of organic matter and en- indicated by the highest IRD values, the absence of benthic assemblage hanced oxygen consumption in bottom waters and sediments (Fig. 9). and zero PFNs (Figs. 4a, b and 7b). Similar conditions — evidenced by Maxima of the C/N ratio during TV and TI (Allerød and Boreal) the extremely low BFNs and PFNs — likely took place during the latest (Fig. 7f) indicate a larger supply of terrestrial organic matter at our MIS 2 at ~15.7 and ~14.8 kyr BP (Oldest Dryas) (Fig. 4a, b). During the core location than further south (Iwasaki et al., 2012; Seki et al., 2004; remainder of the latest MIS 2, prolonged seasonal ice cover and low sur- Ternois et al., 2001). Apparently, erosion of the northwestern Okhotsk face productivity prevailed, as indicated by the A. angulosa assemblage Sea shelves during the last and previous deglacial sea level rises provid- and by a slightly lower IRD values and higher BFNs, PFNs, color b*, bio- ed enhanced offshore supply of terrestrial organic matter via the ice genic opal and CaCO3 contents compared to the latest MIS 12 (Figs. 4a, b rafting and surface currents, as well as via the downslope tidal currents and 7b–d). The A. angulosa assemblage points to the existence of the and intermediate water (Nürnberg and Tiedemann, 2004; Seki et al., northern ice shelf that is consistent with the previous reconstructions 2004; Ternois et al., 2001). The effective downslope transport during of a perennial ice cover in the northwestern Okhotsk Sea during MIS 2 TV and TI might have provoked the delivery of organic matter both ter- by means of the IRD data (Gorbarenko et al., 2004, 2010; Nürnberg restrial and marine in origin from the upper slope and shelf. This corrob- et al., 2011). Our findings indicate that the latest MIS 2 conditions orates the assumption about the impact of the downslope supply of have been less severe than those of the latest MIS 12. This agrees with organic matter on the OMZ intensifications inferred from the deglacial the generally accepted less lowered insolation (Fig. 6g) and less exten- benthic assemblages (Section 5.1). Higher C/N ratios during TV and sive ice sheet in the Northern Hemisphere during MIS 2 than during MIS higher IRD during TVa imply an even larger offshore supply of organic 12 (Berger and Loutre, 1991; Vázquez Riveiros et al., 2013). matter as compared to TI (Fig. 7f). This might have been forced by In both latest glacial periods, the foraminiferal data and overall low more severe glacial conditions during thee latest MIS 12 than during contents of the biogenic components suggest that productivity, flux of the latest MIS 2 (Section 5.2). organic matter and oxygen consumption were substantially reduced, The highest BFNs, BFAR, TOC contents and TOC AR during TV (427 or which may have caused well-oxygenated low intermediate water 425 to 419 kyr BP) and TI (Allerød and Preboreal) (Figs. 4aand7e) sug- and the absence of the OMZ (Fig. 9). During the latest MIS 2, the gest high surface and export productivity and thus support its leading A. angulosa assemblage and highest Uvigerina spp. δ13C values suggest role in the OMZ intensifications in agreement with treatment of the that enhanced formation of the DSW and OSIW further oxygenated deglacial benthic assemblages (Section 5.1). However, relatively low low intermediate water. This corroborates the hypothesis of enhanced PFNs, PFAR and CaCO3 contents at those times, except for the Preboreal OSIW formation during Heinrich Stadial 1 inferred from low ventilation (Figs. 4band7d), would rather imply reduced surface (carbonate) pro- ages (shown in Fig. 8b) and high epibenthic foraminiferal δ13C values in ductivity. In the Okhotsk Sea, coccolithophorids flourished during early core SO178-13-6 (Max et al., 2014). This also agrees with the assump- deglaciations prior to MIS 1, 5, 9 and 11 (Iwasaki et al., 2012; Seki et al., tion that well-ventilated (better than at present) upper intermediate 2004, 2009; Ternois et al., 2001). Our proxies apparently underestimate water existed in the Okhotsk Sea during latest MIS 2 and Last Glacial the surface carbonate production, which might have been due to en- Maximum, which has been proposed on the basis of increased percent- hanced carbonate dissolution and terrigenous dilution in sediments, as ages of radiolarian Cycladophora davisiana — a proxy of well-oxygenated seen in increased B/P ratios and Frag ratios during TVa and increased upper OSIW (from 200 to 500 m water depths) (Gorbarenko et al., IRD during TVa and TIa (Allerød) (Figs. 4c, e and 7d). On the other 2010; Itaki et al., 2008; Matul et al., 2015). Matul (2009) interpreted hand, the highest BFNs, BFAR, TOC contents and TOC AR may partly re- high percentages of C. davisiana during MIS 2 (up to 50%) and MIS 12 sult from the downslope supply of organic matter. During TV (427 or (up to 40%) in core MD01-2415 (studied here) as an indicator of 425 to 419 kyr BP), higher BFNs and TOC contents, as well as the benthic strengthened (more than at present) formation of the upper OSIW. assemblage composition and moderate preservation (Section 5.1)sup- These results suggest that formation of the DSW and OSIW caused the port even higher surface and export productivity likely arising from a better oxygenation of low intermediate water and the disappearance larger offshore supply of organic matter compared to TI (Allerød and of the OMZ not only during the latest MIS 2 but also during the latest Preboreal). MIS 12. At 419–416 and 9–9.5 kyr BP, the decrease in the BFNs, BFAR, TOC contents and TOC AR correlates with the drop in the PFNs, PFAR and

5.3. Gradual intensification of the OMZ during TV and TI CaCO3 contents, and the presence of the B. spissa assemblage (Figs. 4a, b, 6c and 7d, e). The results imply that the OMZ intensifications at the In the Okhotsk Sea, assemblages with abundant B. spissa have been end of both terminations was not controlled by high surface and export described in TI sediments (Bubenshchikova et al., 2010; Gorbarenko productivity but was maintained by additional factors (e.g., inflow of the et al., 2002a; Gorbarenko et al., 2010) but were found neither in recent DPW, as discussed later). Changes in the surface water may also have sediments nor in TII, TIII or TIV sediments (Saidova, 1961, 1997; Barash played a role. Similar to the present, two microplankton blooms et al., 2001, 2006; Khusid et al., 2005; our unpublished data for core (Fig. S1) were developed in the Okhotsk Sea during the late deglacial MD01-2415). The results of the present study reveal that the B. spissa to full interglacial times of MIS 1, 5, 9 and 11 (Iwasaki et al., 2012; assemblage was distributed during both TV and TI (Fig. 6a, c). Notewor- Seki et al., 2004). We suppose that enhanced surface freshening during thy that the duration of TV is markedly longer than that of TI (15 kyr vs. autumns at 419–416 and 9–9.5 kyr BP limited the carbonate production 6.7 kyr, respectively, in core MD01-2415) (Fig. 6). The exceptional long and led to seasonal stagnation of the water column. The latter along duration of TV compared to other terminations has been attributed to with the downslope supply of organic matter, recorded by the C/N the weak insolation increase that resulted in the prolonged decline of ratio peaks at ~417.5 and ~9 kyr BP and the maxima of the V. sadonica the Northern Hemisphere ice sheet and the slow recovery of North At- and B. subspinescens percentages at ~9 kyr BP (Figs. 5d, g and 7f), main- lantic Meridional Circulation (Vázquez Riveiros et al., 2013). During tained the OMZ intensifications. Reconstructions of the regional climate .Bbnhhkv ta./Mrn irplotlg 2 21)52 (2015) 121 Micropaleontology Marine / al. et Bubenshchikova N.

Table 5 Environmental preferences of the dominant benthic foraminiferal species in core MD01-2415.

Species Water masses in the Okhotsk Seaa and bottom water References Food, bottom and pore water oxygenation and carbonate corrosiveness, sediments and References oxygen conditionsb microhabitats

Angulogerina angulosa DSW-upper OSIW 1 associated with active currents providing suspended OM, 4, 6, 7, 11, 14, 18 (and related species) DSW-OSIW-DPW 2 organic-poor (0.1-0.8%) coarse sediments and suboxic to high this well-oxygenated bottom waters 4, 11 oxic conditions study related to the ice shelf edge conditions with active currents, 23 low organic matter (OM) ﬂux and coarse sediments shallow infaunal 8, 9, 13

Uvigerina auberiana DPW-upper KBW, 1, 2 related to low to high surface productivity and OM flux 12, 19 (and some hispid uvigerinids) OMZ and below OMZ inhabits a wide range of organic-enriched (0.3-5.2%) sediments 10, 19 suboxic to low this shows a better adaptation to a somewhat lower OM flux, and 10, 21, 28, 29 oxic conditions study slightly more oxygenated bottom water at deeper locations and/ or below OMZs 2, 10, 19, 29 tolerates prolonged duration of the seasonal sea ice cover and pulsed OM flux 28 shallow infaunal 13, 28

Uvigerina akitaensis DSW-OSIW-DPW- 1 related to high SP and high and sustained OM ﬂux 8, 10, 12, 14,17-19, 24, – 27-29 69 (and some Uvigerina peregrina-like upper KBW responds to input of the phytodetritus in meso-eutrophic areas 24, 26, 27 species) OSIW-DPW-KBW 2 avoids the most eutrophic and strongly oxygen-depleted areas 5, 10, 12, 17 suboxic to high this inhabits a wide range of organic-enriched (0.3-5%) sediments 8, 10, 18, 19, 27 oxic conditions study shallow infaunal 8, 9, 13, 24, 26-28

Takayanagia delicata DPW and OMZ 1 adapted to high SP, high and sustained OM ﬂux, moderate to strong oxygen depletion in 8, 12, 27 OSIW-DPW 2 bottom waters in areas with well-developed OMZ, dwells in organic-rich (2-5%) sediments suboxic to high 3 responds to input of the phytodetritus in meso-eutrophic areas 27 oxic conditions shallow infaunal 8, 27, 28

Bolivina spissa DPW and OMZ 1, 2 adapted to high SP, high and sustained OM ﬂux, moderate to strong oxygen depletion in 5, 8, 12, 16, 27 dysoxic conditions 3 bottom waters in areas with well-developed OMZ, dwells in organic-rich (2-5%) sediments responds to high input of the phytodetritus in meso-eutrophic areas 16, 26, 27 shallow infaunal 8, 16, 26, 27

Brizalina subspinescens DPW and OMZ 1, 2 tolerates high and sustained ﬂux of degraded and refractory OM originated from high SP 13, 28 dysoxic conditions 3 and downslope supply

(continued on next page) 63 64 Table 5 (continued)

Species Water masses in the Okhotsk Seaa and bottom water References Food, bottom and pore water oxygenation and carbonate corrosiveness, sediments and References oxygen conditionsb microhabitats

shallow to intermediate infaunal 13, 28

Valvulineria sadonica OSIW-DPW-KBW 1, 2 tolerates high and sustained ﬂux of degraded and refractory OM originated from high SP 13, 28 (Valvulineria spp.) suboxic to high this and downslope supply oxic conditions study intermediate infaunal 13, 28

Islandiella norcrossi OSIW-DPW 1, 2 associated with moderate to high and seasonal SP and OM ﬂux in seasonally ice free areas 22, 25, 28 suboxic to high this of the northern high latitudes, inhabits gently organic-enriched (0.4-2.0%) sediments 22 oxic conditions study shows opportunistic response on input of the phytodertritus in meso-oligotrophic areas 25 shallow infaunal 20, 28

Oridorsalis umbonatus OSIW-DPW 1 related to low to moderate and seasonal SP and OM ﬂux, 6, 10, 11,13-14, 18, 19-22, 29 (Oridorsalis spp.) suboxic to high this well-oxygenated bottom and pore waters and 6, 11, 14, 18, 19, 29 oxic conditions study moderately carbonate corrosive bottom and pore waters between lysocline and CCD 18, 19 52 (2015) 121 Micropaleontology Marine / al. et Bubenshchikova N. responds to deposition of the phytodertritus in oligotrophic areas 15, 30 inhabits organic-poor (0.1-0.7%) and carbonate-bearing sediments 6, 11, 13, 18, 22, 30 shallow to intermediate infaunal 8, 13, 20

Pullenia bulloides KBW 1 related to low to moderate and seasonal SP and OM ﬂux, 13, 14, 18, 19, 21, 22, 29 low oxic this well-oxygenated bottom and pore waters waters and 18, 19, 29 conditions study moderately carbonate corrosive bottom and pore waters between lysocline and CCD 14, 18, 19 dwells in gently organic-enriched (0.4-2.0%) sediments 6, 19, 22 shallow infaunal 8, 9, 13, 20

Miliammina herzensteini DSW-OSIW-DPW 1 adapted to moderate to high and seasonal SP and OM ﬂux, highly carbonate 4, 7, 8, 11, 14, 23 (Miliammina spp.) undersaturated bottom and pore waters above and below CCD inhabits gently organic-enriched (0.5-1%) diatomaceous oozes 23

Karreriella baccata OSIW-DPW 1 tolerates low and seasonal SP and OM ﬂux, highly carbonate undersaturated bottom and pore 10, 11 (Karreriella spp.) DPW-upper KBW 2 waters above and below CCD Martinottiella communis not found 1, 2 10, 11, 14, 18

References: 1 — Saidova, 1961;2— Inoue, 1989;3— Kaiho, 1994;4— Milam and Anderson, 1981;5— Mullins et al., 1985;6— Mackensen, 1985;7— Ward et al., 1987;8— Mackensen and Douglas, 1989;9— Corliss, 1991;10— Hermelin and Shimmield, 1990;11— Mackensen et al., 1990;12— Sen Gupta and Machain-Castillo, 1993;13— Rathburn and Corliss, 1994;14— Mackensen et al., 1995;15— Gooday, 1996;16— Silva et al., 1996;17— Bernhard et al., 1997;18— Harloff

— — — — — — — – and Mackensen, 1997;19 Schmiedl et al., 1997;20 Wollenburg and Mackensen, 1998a, 1998b;21 Altenbach et al., 1999;22 Rytter et al., 2002;23 Ishman and Szymcek, 2003;24 Fontanier et al., 2003;25 Wollenburg et al., 69 2004;26— Nomaki et al., 2006;27— Shepherd et al., 2007;28— Bubenshchikova et al., 2008;29— De and Gupta, 2010;30— Gooday et al., 2010. a See text for the water mass abbreviations. − − − − b Dysoxic (0.1–0.3 ml l 1), suboxic (0.3–1.5 ml l 1), low oxic (1.5–3mll 1) and high oxic (3–6mll 1) groups following Kaiho (1994). N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69 65 conditions (Bazarova et al., 2011; Melles et al., 2012) suggest that en- and enhanced decay of organic matter (Gorbarenko et al., 2010; Itaki hanced surface freshening around our core location at 419–416 and et al., 2008). On the other hand, reduced formation of the OSIW during 9–9.5 kyr BP could have been due to increased precipitation and fluvial the Bølling–Allerød and early Holocene has been used to explain in- discharge (i.e. the Amur River, northern coast rivers and Kamchatka creased ventilation ages in core LV29-114-3 (shown in Fig. 8b) and rivers). lowered epibenthic δ13C values in core SO178-13-6 (Max et al., 2014). During TVb, higher biogenic opal contents and lower MS and IRD In our core MD01-2415, the C. davisiana percentages increase up to values coincide with an early appearance of the phytodetritus-related 80% during TI and decrease down to 20% during TV, representing I. norcrossi assemblage (Figs. 6d and 7a–c), suggesting larger biogenic strengthened and reduced (but still more than at present) formation opal production due to shorter seasonal ice cover as compared to TIb. of the upper OSIW, respectively (Matul, 2009). These results do not ex- This was likely provoked by more enhanced inflow of a relatively clude a reduced formation of the DSW and OSIW and its increasing im- warm NPW into the southeastern Okhotsk Sea during TVb than during pact on the OMZ intensification during TVb, which is in line with a TIb. As at the present-day Okhotsk Sea, both intermediate water and flu- stronger OMZ based on the B. spissa assemblage composition vial runoff have increased silicate contents (Freeland et al., 1998; (Section 5.1). Shesterkina and Talovskaya, 2010), the larger biogenic opal production The DPW originates from low intermediate water circulated in the during TVb could have also occurred as a result of more intense upwell- Subarctic Gyre (Fig. 1b, c). The results of our study imply that an in- ing near the Kashevarov Bank or a larger fluvial discharge. During TVb, crease of surface productivity and a weakening of ventilation near the higher precipitation (Melles et al., 2012) could have led to larger fluvial site of the DPW source waters could have caused expansion of more discharge and in turn to more intense seasonal stagnation relative to oxygen-depleted and older DPW and intensification of the OMZ during TIb. This agrees with a stronger OMZ during TVb than during TIb, as TVb and TIb in the Okhotsk Sea. Evidence of strengthened OMZ condi- seen in the B. spissa assemblage compositions (Section 5.1). tions during the Bølling-Allerød and early Holocene has been presented During TIb, a lower Uvigerina spp. δ13C value implies weaker ventila- for both the East and Subarctic Pacific. However, the majority of the tion relative to TIa (Fig. 8a) and thus support the scenario of the en- studies, reconstructed a stronger OMZ due to higher surface productiv- hanced inflow of the DPW, leading to the OMZ intensification, as ity and weaker ventilation during the Bølling–Allerød in comparison to indicated by the B. spissa assemblage (Section 5.1). However, the avail- early Holocene (Cannariato and Kennett, 1999; Crusius et al., 2004; able ventilation ages for TI and Uvigerina spp. δ13C data for TV (Fig. 8) Sagawa and Ikehara, 2008; Shibahara et al., 2007). An exception is point to weaker ventilation during early deglacial stages compared to weaker ventilation during the early Holocene than during the Bølling– later ones. The reasons for the discrepancy between the proxies are un- Allerød recorded off the coast of Vancouver Island (McKay et al., clear. Information on ventilation changes in the Okhotsk Sea during TV 2005). It has been shown that in the Subarctic Pacific higher surface pro- and TI is indeed sparse. According to the C. davisiana-based studies, ductivity characterized the early stages of TI to TV as compared to the well-ventilated upper OSIW during TIa and TIb co-existed with subsequent latest deglacial to interglacial intervals (Gebhardt et al., oxygen-poor low intermediate water caused by both weak ventilation 2008). The discrepancy between the Okhotsk Sea and North Pacific

Fig. 9. Scheme, representing close similarity in glacial to interglacial environmental changes in the Okhotsk Sea over the latest MIS 12–11c and latest MIS 2–1. OM = organic matter. PP = primary production. See text for further explanations. 66 N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69 mentioned above suggests expansion of the DPW as the only contribut- ocean carbonate saturation known as the Mid-Brunhes Dissolution In- ing factor for the OMZ intensifications during TVb and TIb in the terval (MBDI) centered around MIS 11 (Barker et al., 2006; Hodell Okhotsk Sea. et al., 2003). During MIS 1, larger biogenic opal and carbonate production are consistent with higher organic matter flux and less pro- 5.4. Weakening of the OMZ during early MIS 11c and MIS 1 nounced weakening of the OMZ as compared to early MIS 11c, as evidenced by the I. norcrossi assemblage compositions Assemblages with dominant I. norcrossi (also cited as Discoislandiella (Section 5.1). smechovi or Cassidulina teretis) have frequently been found in the recent Following TI, the Uvigerina spp. δ13C data indicate somewhat and MIS 1 sediments of the Okhotsk Sea (Bubenshchikova et al., 2010; enhanced ventilation throughout MIS 1, except for at ~2–2.2 kyr BP Gorbarenko et al., 2002a; Gorbarenko et al., 2010; Saidova, 1997). In (Fig. 8a). The available ventilation ages imply that ventilation weak- core MD01-2415, the I. norcrossi assemblage documents the resembling ened at 6.1 and 2.4 kyr BP and strengthened at 6.0, 5.7, 3.5 and 0.2 kyr OMZ weakenings during full interglacial times of MIS 11c and MIS 1 BP (Fig. 8b). Despite of a weak correlation between the proxies, the (Figs. 6dand9). Because of the presence of carbonate dissolution events results do not exclude a possibility of the water mass changes (an in- in sediments of the older interglacial (discussed in Section 5.5), the crease in the formation of the DSW and OSIW and a reduction in the weakened OMZ existed for a shorter time during MIS 11c (7.8 kyr) DPW inflow) and their contribution to the weakening of the OMZ than during MIS 1 (8.5 kyr), although the duration of MIS 11c is longer during early to middle MIS 1. Following TV, the Uvigerina spp. δ13C than that of MIS 1 (21 vs. 8.5 kyr, respectively, in core MD01-2415) data are scarce for any speculations about ventilation changes (Fig. 6). The long duration of MIS 11c and the orbital similarity between (Fig. 8a). According to the C. davisiana-based studies, the formation MIS 1 and MIS 11c has been related to a minimum of the 400–kyr eccen- of the upper OSIW increased during early to middle MIS 1, while it tricity cycle falling on both interglacials (Loutre and Berger, 2003). The weakened during late MIS 1 (at 3–1 kyr BP) in the Okhotsk Sea I. norcrossi assemblage at 414.5–413.2, 411.5–405 and 8.5–0 kyr BP co- (Gorbarenko et al., 2010; Itaki et al., 2008). In our core MD01-2415, occurs with the highest PFNs, PFAR, color b*, biogenic opal and CaCO3 the relative abundance of C. davisiana decreases from 80 to 20% during contents (Figs. 4b, 6dand7a–d). The assemblage coincides with periods MIS 1 and from 20 to 5% during MIS 11c, indicating reduced (but still of high standings of the global sea level (Fig. 6f). We suggest that after more than at present) and almost ceased formation of the upper stabilization of the global sea level, the increased biogenic opal and car- OSIW, respectively (Matul, 2009). These reconstructions corroborate bonate production in the surface water similar to the present caused the scenario of enhanced DSW and OSIW formation, contributing to moderate to high and pulsed flux of organic matter that resulted in the weakening of the OMZ during early to middle MIS 1, while they the weakened oxygen consumption and OMZ (Fig. 9). do not support it during early MIS 11c. It remains to be determined Following TV and TI, the C/N ratio decrease (Fig. 7f) that indicates whether the OMZ during MIS 11c and MIS 1 was influenced by ventila- the cessation of a large offshore organic supply and its influence on tion changes. the surface productivity and OMZ. Nevertheless, a weak downslope supply of organic matter due to continuing shelf erosion took place during early MIS 11c, as evidenced by a slightly increased C/N ratio, MS, IRD 5.5. Carbonate dissolution events during MIS 11c and TOC contents (Fig. 7a, b, e, f) and the I. norcrossi assemblage composition (Section 5.1). This assumption is supported by the generally ac- Strong carbonate dissolution in sediments has occurred at cepted higher state of the global sea level during MIS 11c than during 416–414.5, 413.2–411.5 and 405–395 kyr BP, as evidenced by the MIS 1 (Figs. 2cand6f). badly-preserved “agglutinated” assemblage, high B/P and Agl/Cal ratios,

BFNs, BFAR, TOC contents and TOC AR increased but did not reach as well as low or zero BFNs, PFNs and CaCO3 contents (Figs. 4a–d, 6eand deglacial values at 414.5–413.2, 411.5–405 and 8.5–0 kyr BP (Figs. 4a 7d). During these events, the biogenic opal production by the color b* and 7e), suggesting less than deglacial surface and export productivity and biogenic opal contents (Fig. 7c) was close to that of the remainder at times of weakened OMZ. On the other hand, the data may reflect of MIS 11c (except for at 400–395 kyr BP, as noted below). We suppose the cessation of both flourishing of the carbonate microplankton (likely that the formation of these dissolution events may be related to an in- underpreserved in our core) and a large downslope supply of organic terruption of the surface carbonate production and increase of the matter during early MIS 11c and MIS 1 as compared to preceding bottom water carbonate corrosiveness. An enhanced freshwater supply deglacial intervals. via precipitation and fluvial discharge during autumns could have During MIS 1, slightly lower MS and IRD associated with higher caused the interruption of the carbonate production at 416–414.5,

PFNs, PFAR, color b*, biogenic opal and CaCO3 contents (Figs. 4band 413.2–411.5 and 405–400 kyr BP. A maximum of precipitation over 7a–d) imply that shorter seasonal ice cover favored larger biogenic the North-East Siberia at 405–400 kyr BP (Melles et al., 2012) partly cor- opal and carbonate production as compared to early MIS 11c. In our roborates this scenario. Prolonged seasonal ice cover might have sup- opinion, more enhanced inflow of the NPW into the Okhotsk Sea and pressed not only carbonate but also biogenic opal production at more intense upwelling near the Kashevarov Bank maintained larger 400–395 kyr BP, as indicated by increased MS and IRD values and biogenic opal production during MIS 1, while less intense upwelling lowered color b* and biogenic opal contents (Fig. 7a–c) in line with a and larger fluvial discharge led to moderate biogenic opal production drop in insolation (Fig. 6g) and onset of glacial conditions. In addition, during early MIS 11c. We also suggest that moderate autumn surface a drawdown of the global ocean carbonate saturation during MBDI, freshening due to increased precipitation and fluvial discharge restrict- which occurred with maximal intensity around ~400 kyr BP (Barker ed the carbonate production during early MIS 11. Recent regional stud- et al., 2006; Hodell et al., 2003), might have forced both the interruption ies show lower precipitation and higher biogenic opal production due to of the surface carbonate production and inflow of more carbonate- more intense upwelling of the Pacific Deepwater in the Subarctic Pacific corrosive DPW into the Okhotsk Sea during the dissolution events. As during MIS 1 than during MIS 11c (Gebhardt et al., 2008; Melles et al., a result of the above-mentioned causes, bottom and pore waters in

2012)thatfits with our above suggestions. In addition, lowered CaCO3 sediments became more unsaturated with respect to CaCO3 than at contents in early MIS 11c sediments may be a result of enhanced car- present. This may indicate shoaling of the CCD or, alternatively, deepen- bonate dissolution, which is in line with the I. norcrossi assemblage com- ing of the upper depth of the carbonate accumulation toward the stud- position (Section 5.1). The enhanced dissolution could be amplified by ied site (at 822 m water depth). At present, the carbonate accumulation reduced surface carbonate production and inflow of more carbonate- occurs between 500 and 1500 m water depths on the northern slope of corrosive DPW into the Okhotsk Sea from the North Pacific. These the Okhotsk Sea (see Section 2). Whether the dissolution events are changes in turn might have been forced by a drawdown of the global local or regional merits further study. N. Bubenshchikova et al. / Marine Micropaleontology 121 (2015) 52–69 67

6. Summary and conclusions Appendix A. Supplementary data

The benthic foraminiferal assemblage compositions in combination Supplementary data associated with this article can be found in the with other proxy records of core MD01-2415 reveal close similarity on-line version and on www.pangaea.de,athttp://dx.doi.org/10.1016/ between the latest MIS 12–11c and latest MIS 2–1 in the Okhotsk Sea. j.marmicro.2015.09.004. These data include Google maps of the most The orbitally forced similar changes in the global sea level might have important areas described in this article. been responsible for the resembling gradual OMZ intensifications during TV and TI and subsequent weakenings of the OMZ during MIS 11c References and MIS 1. The sea level changes controlled the offshore supply of fl organic matter and surface productivity, which in turn regulated the in- Altenbach, A.V., P aumann, U., Schiebel, R., Thies, A., Timm, S., Trauth, M., 1999. Scaling percentages of benthic foraminifera with flux rates of organic carbon. J. Foraminifer. tensity and seasonality of organic matter flux and oxygen consumption Res. 29 (3), 173–185. in bottom waters and sediments. Arzhanova, N.V., Naletova, I.A., Sapozhnikov, V.V., Polyakova, A.V., 2002. Phytoplankton (1) Disappearance of the OMZ at times of low productivity is evi- provision with nutrient stocks in the northern part of the Sea of Okhotsk. Oceanology 41 (5), 723–735 (English Translation of Okeanologiya by MAIK, Nauka/Interperiodica denced by the absence of benthic assemblages during the latest MIS Publishing, Russia). 12 (435 to 429 BP) and the A. angulosa assemblage during the latest Barash, M.S., Bubenshchikova, N.V., Kazarina, G.K., Khusid, T.A., 2001. Paleoceanography MIS 2 (18 to 14.7 kyr BP). Increased IRD or MS indicate almost perennial of the central part of the Sea of Okhotsk over the past 200 kyr (on the basis of micropaleontological data). Oceanology 41 (5), 755–767 (English Translation of ice cover with periods of active ice rafting during the latest MIS 12 and Okeanologiya by MAIK, Nauka/Interperiodica Publishing, Russia). prolonged seasonal ice cover during the latest MIS 2. The A. angulosa Barash, M.S., Matul, A.G., Kazarina, G.K., Khusid, T.A., Abelmann, A., Biebow, N., Nürnberg, assemblage, high Uvigerina spp. δ13C values and previously published D., Tiedemann, R., 2006. Paleoceanography of the central Sea of Okhotsk during middle Pleistocene (350–190 ka) as inferred from micro-paleontological data. Oceanolo- low ventilation ages suggest that enhanced formation of the DSW and gy 46 (4), 501–512 (English Translation of Okeanologiya by Pleiades Publishing, Inc.). OSIW in the Okhotsk Sea further oxygenated low intermediate water Barker, S., Archer, D., Booth, L., Elderfield, H., Henderiks, J., Rickaby, R.E.M., 2006. Globally during the latest MIS 2. increased pelagic carbonate production during Mid-Brunhes dissolution interval and – fi the CO2 paradox of MIS 11. Quat. Sci. Rev. 25, 3278 3293. (2) Gradual intensi cation of the OMZ is reconstructed by the Bazarova, V.B., Mokhova, L.M., Klimin, M.A., Kopoteva, T.A., 2011. Vegetation development U. akitaensis assemblage during early TV (429 to 422 kyr BP) and and correlation of Holocene events in the Amur River basin, NE Eurasia. Quat. Int. 237 early TI (14.7 to 12.8 kyr BP) and the B. spissa assemblage during late (1–2), 83–92. TV (422 to 416 kyr BP) and late TI (11.7 to 8.5 kyr BP). Maxima of the Berger, A., Loutre, M.F., 1991. Insolation values for the climate of the last 10 million years. Quat. Sci. Rev. 10, 297–317. C/N ratio during TV and TI (Allerød and Boreal), as well as high BFNs, Bernhard, J.M., Sen Gupta, B.K., Borne, P.F., 1997. Benthic foraminiferal proxy to estimate BFAR, TOC contents and TOC AR during TV (427 or 425 to 419 kyr BP) dysoxic bottom water oxygen concentrations: Santa Barbara Basin, U.S. PacificConti- – and TI (Allerød and Preboreal) corroborate the assumption that a high nental Margin. J. Foraminifer. Res. 27 (4), 301 310. fl Bezrukov, P.L., 1960. The Bottom Sediments of the Ohkhotsk Sea. (Donnie otlozhenia and sustained ux of organic matter from primary production and Ohotskogo morya). 32. Akademiya Nauk SSSR Publishing, Moscow, pp. 15–96 downslope supply led to the similar gradual OMZ intensifications. The (Trudi Instituta Okeanologii). B. spissa assemblage points to enhanced inflow of oxygen-depleted Biebow, N., Kulinich, R., Baranov, B., 2002. KOMEX II, Kurile Okhotsk Sea Marine Experi- fi ment: cruise report RV Akademik M. A. Lavrentyev cruise 29, leg 1 and leg 2, in: DPW, fostering the OMZ intensi cations during late TV and late TI. The GEOMAR Report 110, edited by. In: Biebow, N., Kulinich, R., Baranov, B. (Eds.), Leibniz 13 Uvigerina spp. δ C data, as well as our own and previously published Institute of Marine Sciences, Kiel. ventilation ages only partly support this scenario, as they indicate weak- Bogdanov, K.T., Moroz, V.V., 2004. The Kuril–Kamchatka Current and Oyashio Current Water (Vodi Kurilo-Kamchatskogo Techenya i Techenya Oiasio). Dalnauka, Vladivos- ened ventilation during early TV, early TI and late TI. tok (141 pp. (in Russian)).

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