Rapid Sea-Level Rise and Holocene Climate in the Chukchi Sea

Rapid sea-level rise and Holocene climate in the Chukchi Sea

Lloyd D. Keigwin Jeffrey P. Donnelly Mea S. Cook Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Neal W. Driscoll Scripps Institution of Oceanography, La Jolla, California 92093, USA Julie Brigham-Grette Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA

ABSTRACT Three new sediment cores from the Chukchi Sea preserve a record of local paleoenvironment, sedimentation, and flooding of the Chukchi Shelf (ϳ؊50 m) by glacial-eustatic sea-level rise. Radiocarbon dates on foraminifera provide the first marine evidence that the sea invaded Hope Valley (southern Chukchi Sea, ؊53 m) as early as 12 ka. The lack of significant sediment accumulation since ca. 7 ka in Hope Valley, southeastern Chukchi Shelf, is consistent with decreased sediment supply and fluvial discharge to the shelf as deglaciation of Alaska concluded. Abundant benthic foraminifera from a site west of Bar- row Canyon indicate that surface waters were more productive 4–6 ka, and this productivity varied on centennial time scales. An offshore companion to this core contains a 20 m record of the Holocene. These results show that carefully selected core sites from the western Arctic Ocean can have a temporal resolution equal to the best cores from other regions, and that these sites can be exploited for high-resolution studies of the paleoenvironment.

Keywords: sea level, Chukchi Sea, Holocene, Bering Strait, foraminifera.

INTRODUCTION during sea-level rise, indicates there must have row Canyon off Point Barrow, Alaska, a major The Chukchi Sea overlies part of the broad been a significant increase in fluvial discharge conduit for dense briny waters that are pro- circum-Arctic continental shelf that was ex- during deglaciation. This might indicate the duced on the shelf during winter, and sediment posed when sea level fell during the Last Gla- presence of significant continental ice in Alas- that is resuspended by autumn and early win- cial Maximum (LGM). Understanding the his- ka during the LGM. Here we report on sedi- ter storms (Weingartner et al., 1998). Our sed- tory of sea level and climate in the Chukchi mentological, geochemical, and paleontologi- iment cores from Hope Valley (HLY0204 Sea is important because when sea level was cal results from shelf and slope locations in 01GGC/02JPC, 45 m) and the shallow site low the climate was more continental across the Chukchi Sea. Hope Valley crosses the near Barrow Canyon (HLY0205 19GGC, 369 a vast region, and when sea level is high the Chukchi Shelf and has a bathymetric expres- m) contain high-resolution records of Holo- flow of North Pacific water through Bering sion in the nearshore regions of Kotzebue cene climate, sea-level change, and the history Strait (sill depth 50 m) affects the fresh water Sound (Fig. 1). It most likely drained a por- of sediment source and sink for the Chukchi and nutrient balance of the Arctic (Aagaard tion of the continent, yet it does not connect Shelf. Preliminary results from the deeper and Carmack, 1989; Woodgate and Aagaard, to the offshore drainage network described by slope (HLY0205 15/16JPC, ϳ1300 m) indi- 2005). It has been traditionally thought that Hill et al. (2005). Our slope sites are near Bar- cate the potential for very high resolution pa- humans (and other fauna) populated the leoclimate studies in the Chukchi Sea. Americas by migrating across the exposed continental shelf prior to flooding of the strait; METHODS however, this view has been challenged by ev- Cores were sampled at 10 cm spacing for idence for a maritime route (Dalton, 2003; identifying the foraminiferal fauna, for dating, Sarnthein et al., 2006). Previous work, based and for stable isotopes using methods de- mostly on terrestrial organic macrofossils scribed by Cook et al. (2005). Abundance identified as peat, suggests that Bering Strait peaks in benthic foraminifera were sampled at was flooded ca. 11,000 yr B.P. (Elias et al., 1–2 cm spacing for 14C dating using standard 1992, 1996). methods at the National Ocean Sciences Ac- During summer 2002, we surveyed and celerator Mass Spectrometer facility at Woods cored many locations in the Bering (Cook et Hole. Radiocarbon dates were calibrated to al., 2005) and Chukchi Seas (Hill et al., 2005) calendar years using CALIB v.5.x (Stuiver et to study climate and sea level. Hill et al. al., 1998). Samples were taken at 1–10 cm Figure 1. Chukchi Sea and surrounding re- (2005) showed that the large mismatch be- gions with core locations discussed in this spacing from 02JPC for grain-size measure- tween the size of buried channels on the shelf paper. HV—Hope Valley, BC—Barrow Can- ments using a Beckman Coulter LS13320 la- and modern discharge, and their formation yon, BS—Bering Strait. ser diffraction particle size analyzer.

᭧ 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; October 2006; v. 34; no. 10; p. 861–864; doi: 10.1130/G22712.1; 4 ﬁgures. 861 Figure 3. Stable isotope ratios and faunal counts on upper slope near Bar- row Canyon (A–D) and at mid-slope location (E), and positions of 14C dates (in conventional yr B.P.). As described in text, these changes may indicate centennial-scale changes in sea-surface temperature, sea ice extent, and ﬂow of surface waters through Bering Strait. Shaded bars in A– D illustrate coherent changes in abundance patterns of foraminifera, and in E illustrate interval of ice-rafted sand and clumps of mixed litholo- gy. N.p.s.—Neoglobo- quadrina pachyderma sinistral.

RESULTS carbon isotope ratios of E. excavatum reveal All data will be archived at the World Data that profound environmental changes were as- Figure 2. Paleontological and sedimentolog- Center for Paleoclimatology (Boulder, Colo- sociated with the lithologic change at 850 cm ical (A), stable isotope (B), and sedimentation rate (C) evidence for sea-level change rado). In each of the cores reported on here, in 02JPC (Fig. 2B). At the abundance peak of on Chukchi Shelf. In Hope Valley, sharp con- the Holocene section is mostly featureless clay this species the ␦18O was nearly Ϫ8.0‰, and tact at ~850 cm marks transition in grain and silt. This suggests high accumulation rates assuming a linear sedimentation rate, the ␦18O size (solid squares) from sandy layer that without major redeposition events. Hope Val- increased by 7‰ within ϳ200 yr. Increasing contains terrestrial plant fragments to over- ley core 02JPC contains ϳ850 cm of silt over- ␦18O continued to the core top, but most of lying marine silt (A). Abundance peak in benthic foram Elphidium (open squares) lying a unit of fine sand (Fig. 2A). Elphidium the remaining change occurred before 8 ka. dates to ca. 12 ka (calibrated). Large in- excavatum dominates the foraminiferal fauna, At several hundred meters deeper than crease in ␦18O (solid squares) and decrease but this species is abundant only at about the Hope Valley, on the northwest side of Barrow ␦13 in C (open squares) of this genus indicate 70 cm and 850 cm levels. These peaks in Canyon, core 19GGC has a more diverse fo- sudden change from nearshore brackish conditions to open marine conditions in just abundance were sampled for accelerator mass raminiferal fauna through its 471 cm length. few centuries (B). Declining rate of sedimen- spectrometry (AMS) dating, which indicates The fauna include the planktonic species Neo- tation during late Holocene in both Hope that the silty unit was deposited beginning ca. globoquadrina pachyderma sinistral as well as Valley and on upper slope near Barrow Can- 10,900 Ϯ 140 14C yr B.P. (at 845.5 cm; depth the benthic foraminifera E. excavatum, Noni- yon (C) is consistent with decreased fluvial of sample below mean sea level ϭ 53.5 m). onella labradorica, Cassidulina, and Islan- supply of sediment to Chukchi shelf. In con- trast, sedimentation in deeper water off Bar- The upper peak of E. excavatum abundance diella. On average, these benthic genera com- row Canyon may reflect different sediment dates from 6920 Ϯ 75 yr (at 69.5 cm). (Unless pose Ͼ95% of the benthic fauna. Oxygen transport processes, but with only one date otherwise noted, all dates are in conventional isotope ratios of N. pachyderma and N. labra- we can only estimate minimum Holocene 14C yr.) A companion gravity core (01GGC) dorica display little long-term variability .rate of sedimentation of 220 cm k.y.؊1 from the same depth ϳ2 km away contains through the core, but the abundance of all fo- small peaks in E. excavatum abundance at raminifera decreases from maxima before 4 ka ϳ200 cm, indicating that the piston core may to the core top (Fig. 3 shows only the abun- have overpenetrated by ϳ1 m. Oxygen and dance of N. pachyderma s., Elphidium, and N.

862 GEOLOGY, October 2006 labradorica). AMS dating of four abundance lived in water as deep as 5 m, the mollusc date peaks in E. excavatum and N. labradorica provides only a minimum age for marine in- confirms that the core represents the late Ho- cursion, and the water depth of ϳ53 m in- locene. There is good evidence of centennial- ferred for the Elphidium spike is a maximum scale variability in the fauna between 4 and 6 depth. Because the uncertainties in the coral- ka, and some of that variability appears co- based sea-level reconstructions are ϳ5 m, and herent. For example, all species reach maxima the foraminiferal data may be similar, isostatic in abundance ca. 4.3 ka and ca. 5.3 ka, and rebound of as much as 10 m after 12 ka (cal- foraminifera are uniformly rare ca. 5.2 ka. At ibrated) is possible. This uncertainty needs to 15/16JPC, ϳ1000 m deeper than 19GGC, be constrained by minimum depths based on there may be similar variability in the abun- dated terrestrial deposits. However, we con- dance of N. pachyderma s. Where sampled clude that Bering Strait and most of Chukchi closely, between 1300 and 1500 cm, we find Shelf was flooded by 12 ka (calibrated), at peaks in this species at ϳ1320 (6.0 ka) and least 1 k.y. before previously thought, and that ϳ1450 cm. These and other faunal changes any isostatic rebound from nearby continental near Barrow Canyon are not linked in any ob- ice (Hill et al., 2005) was probably within the ␦18 Ͻ vious way to the O record of either plank- Figure 4. New sea-level data from Chukchi uncertainties of the data in Figure 4 ( 10 m). tonic or benthic foraminifera. Sea (in calibrated yr) plotted with latest U series dates on corals (Bard et al., 1998) Sediment Balance on Chukchi Shelf DISCUSSION from Barbados (open squares) and Tahiti The sedimentary successions in Hope Val- The temporal resolution of faunal and stable (open triangles). Point from 10JPC is mini- ley and on the slope near Barrow Canyon re- isotope results discussed here is significantly mum age-depth estimate because underly- ing terrigenous sediments were not recov- cord completely different processes on Chuk- greater than anything published previously ered (Hill et al., 2005), whereas Hope Valley chi Shelf. Although all three sites have high from the Chukchi Sea. Typical Arctic sedi- point is maximum estimate because it im- sedimentation rates, Hope Valley is probably ments in offshore regions are thought to ac- mediately overlies terrigenous sediments. not a site of active flow and sedimentation to- cumulate at ϳ1 cm k.y.Ϫ1, but Andrews and day (Fig. 2C), whereas from 11 ka to ca. 7 ka Dunhill (2004) described an upper slope site the average sedimentation rate was ϳ220 cm far to the east in the Beaufort Sea that has a In Hope Valley, the dramatic increase in k.y.Ϫ1. It is unusual for greatest sediment ac- rate of ϳ130 cm k.y.Ϫ1. A Chukchi Shelf edge ␦18OofE. excavatum must reflect increased cumulation to occur during sea-level rise core with moderate accumulation (10 cm salinity, bearing in mind that the isotope effect Ϫ1 (Christie-Blick and Driscoll, 1995), but it is k.y.Ϫ1) was described by Darby et al. (2001) of temperature is ϳ0.25‰ ЊC , and the sea- consistent with the scenario described by Hill and de Vernal et al. (2005), but accumulation level effect was ϳϪ1.0‰ (Fig. 2B). The in- 13 et al. (2005). The very high sedimentation at Hope Valley, and slope locations near Bar- crease in ␦ C is consistent with a change row Canyon, can be more than 20 times great- from estuarine to open ocean conditions, as is rates during deglaciation may be evidence of er (Fig. 3). Although we provide only one new the abrupt decrease in grain size (Fig. 2A). both a source of sediment on land and a large data point for sea-level reconstruction, this is These three lines of evidence indicate flooding fresh water supply to carry it seaward (glacial the first case where the evidence for flooding of the region by marine waters ca. 12 ka (cal- ice according to Hill et al., 2005). High riv- of Chukchi Shelf has been directly dated. A ibrated) (11,261–12,371 yr; 0.996 relative area erine discharge might have been maintained marine date is important because it gives a at 2␴), assuming a ⌬R of 300 yr as in the until ca. 7 ka by moister climate in northeast minimum age of transgression. Bering Sea (Cook et al., 2005). Although ⌬R Alaska during the early Holocene (Mann et might have been larger, leading to a younger al., 2002). Sea Level date for opening Bering Strait, we assume that While Hope Valley was nearly nondeposi- The history of Beringia sea level, and spe- as the Ϫ50 m isobath was flooded, it was Be- tional, sites near Barrow Canyon were actively cifically that of Bering Strait, has been of in- ring Sea water that invaded Chukchi Shelf accumulating because Barrow Canyon drains terest for decades (Hopkins, 1967). Tradition- (Fig. 1). the Chukchi Shelf. Much of this drainage to- ally, authors used geological markers for The record from Hope Valley is remarkably day is independent of fluvial input. Instead, it shallow water in core samples (e.g., nearshore similar to the results reported by Polyak et al. is driven by brine rejection during sea ice for- foraminiferal faunas or peat), coupled with (2003) from the Russian Arctic near Novaya mation (Weingartner et al., 1998), so as long 14C dates on organic material (McManus and Zemlya. At their shallower site (ϳ35 m), ␦18O as there has been sea ice there has been a Creager, 1984; Elias et al., 1992, 1996). With of E. excavatum increased rapidly by 8‰ ca. mechanism for exporting sediment from the the advent of AMS dating, it is possible to 8 ka as the shoreline migrated landward. Our shelf. An additional export mechanism for directly date terrestrial organic material that is one point cannot define a sea-level curve, but fine-grained sediments is the flow of turbid usually disseminated in cores and requires wet it is consistent with both a 14C-dated marine water associated with storm-driven resuspen- sieving and floating to concentrate it (Elias et mollusc from a core farther offshore (10JPC) sion. Our 14C dates from 19GGC show an av- al., 1992). Thus, there remains some doubt from 55 m (Hill et al., 2005), and with the erage sedimentation rate of ϳ100 cm k.y.Ϫ1 that these pieces of organic material are in estimate from Elias et al. (1996) that Bering prior to ca. 4 ka, with a subsequent decrease situ. In the Chukchi Sea, we specifically tried Strait was flooded by 11 ka. These new data to ϳ60 cm k.y.Ϫ1 until ca. 2 ka, and even low- to core sequences with terrestrial macrofossils, show that relative sea level ca. 12 ka (cali- er than that to present but not nearly as low yet of 17 sites cored in water depths between brated) in Beringia probably did not differ sig- as in Hope Valley. The declining rate of sed- 44 and 107 m, none contained any beds of nificantly from the secular record defined by imentation since the middle Holocene may be terrestrial macrofossils. However, in general Barbados and Tahiti corals (Bard et al., 1998) related to the reduced sediment supply to the our core sites were in deeper water and farther (Fig. 4). There are several uncertainties in this shelf since 7 ka if results at Hope Valley prove offshore than those of Elias et al. (1992). simple analysis; e.g., the corals might have to be typical at other sites on the Chukchi

GEOLOGY, October 2006 863 Shelf. However, less sea ice during middle tows from the warm summer 2004 than the Dalton, R., 2003, The coast road: Nature, v. 422, Holocene warmth (6–2.5 ka, according to de colder summer 2002 (Sharon Smith, Decem- p. 10–12, doi: 10.1038/422010a. Darby, D., Bischof, J., Cutter, G., deVernal, A., Vernal et al., 2005) may have led to less brine ber 2005, personal commun. to Keigwin). Hillaire-Marcel, C., McManus, J., Osterman, rejection, but it also might have facilitated L., Polyak, L., and Poore, R., 2001, New rec- more resuspension of sediment by storms. CONCLUSIONS ord shows pronounced changes in Arctic This process could account for higher accu- The first time-series results from very high Ocean circulation and climate: Eos (Transac- tions, American Geophysical Union), v. 82, mulation rates earlier in the Holocene off Bar- deposition rate cores from the Chukchi Sea p. 601–607. row Canyon. show the potential for developing ocean and de Vernal, A., Hillaire-Marcel, C., and Darby, D.A., climate histories for this region. Our geo- 2005, Variability of sea ice cover in the Chuk- chi Sea (western Arctic Ocean) during the Ho- Climate chemical, micropaleontological, and sedimen- tological study shows the following. (1) Be- locene: Paleoceanography, v. 20, p. PA4018, Neither benthic nor planktonic ␦18O show doi: 10.1029/2005PA001157. much evidence of climate change in Barrow ring Strait was probably flooded between ca. Elias, S.A., Short, S.K., and Phillips, R.L., 1992, Canyon or farther offshore, yet the abundance 11 and 12 ka, and that since that time there Paleoecology of Late-Glacial peats from the Bering Land Bridge: Chukchi Sea Shelf re- of foraminifera is higher in the middle Holo- has been little isostatic rebound. At present this conclusion is based on a large increase in gion: Northwestern Alaska: Quaternary Re- cene than later, and there is good evidence of search, v. 38, p. 371–378, doi: 10.1016/0033- ␦18 systematic change on century time scales (Fig. OinE. excavatum and one AMS date on 5894(92)90045-K. 3). The exact timing of century-scale events this species that gives a minimum date of ma- Elias, S.A., Short, S.K., Nelson, C.H., and Birks, ϳ H.H., 1996, Life and times of the Bering land cannot be addressed further without closer rine incursion at 53 m; more dates of both terrestrial and marine carbon from cores at dif- bridge: Nature, v. 382, p. 60–63, doi: 10.1038/ AMS dates on planktonic foraminifera. Fora- 382060a0. miniferal abundance could be driven by dis- ferent water depths are required to develop a Hill, J.C., Driscoll, N.W., and Donnelly, J.P., 2005, more complete history of relative sea level. solution, but where present, the foraminifera Holocene record of deglaciation on the Chuk- (2) Very little sediment probably accumulates chi Shelf, offshore NW Alaska: Eos (Trans- are generally well preserved, and the more on the Chukchi Shelf today, compared to be- actions, American Geophysical Union), v. 86, solution-susceptible N. pachyderma s. under- fall meeting supplement, abs. OS51B-0567. fore ca. 7 ka, when there was greater terrige- goes abundance changes similar to more ro- Hopkins, D.M., 1967, Quaternary marine transgres- nous sediment input. (3) Higher biological bust benthic genera such as Elphidium. Abun- sions in Alaska, in Hopkins, D.M., The Bering productivity probably accounts for higher fo- Land Bridge: Stanford, California, Stanford dance changes on the slope could also reflect raminiferal abundance near Barrow Canyon University Press, p. 47–90. transport from the shelf, but there are no before ca. 4 ka. It is not known if this resulted Mann, D.H., Peteet, D.M., Reanier, R.E., and Kunz, planktonic foraminifera on the shelf, and El- M.L., 2002, Responses of an arctic landscape from warmer surface waters (less sea ice), or phidium is the dominant benthic genus. In- to Lateglacial and early Holocene climatic greater nutrient supply from Bering Strait. changes: The importance of moisture: Quater- stead it is more likely that the variability re- nary Science Reviews, v. 21, p. 997–1021, flects either production of calcareous ACKNOWLEDGMENTS doi: 10.1016/S0277-3791(01)00116-0. microfossils or their dissolution. Bulk density We thank the officers and crew of U.S. Coast McManus, D.A., and Creager, J.S., 1984, Sea-level of 19GGC gradually decreases ϳ10% from Guard Cutter Healy for use of their ship during data for parts of the Bering-Chukchi shelves of Beringia from 19,000 to 10,000 14C yr. B.P.: bottom to top (ϳ1.7 to ϳ1.5gcmϪ3), as sed- summer 2002, M. Carman, C.E. Franks, S. Pommrehn, and A. Gagnon for technical assistance Quaternary Research, v. 21, p. 317–325, doi: imentation rates decrease (Fig. 2C). Because in the lab, R. Poore for his review of the manuscript, 10.1016/0033-5894(84)90071-1. foraminiferal abundances are maximum deep- and the National Ocean Sciences Accelerator Mass Polyak, L., Stanovoy, V., and Lubinski, D.J., 2003, er in the core, their accumulation rate must Spectrometer facility for providing 14C dates. This Stable isotopes in benthic foraminiferal calcite from a river-influenced Arctic marine environ- have been higher. Because we have argued research was funded by National Science Founda- tion OPP grants 99-1212122 to Keigwin and Dris- ment: Kara and Pechora Seas: Paleoceanog- above against preservational artifacts and coll, and Oak Foundation grant OUSA-02-027 to raphy, v. 18, p. 1003, doi: 10.1029/ transport of foraminifera, we conclude that bi- Donnelly and Cook. 2001PA000752. ological production near Barrow Canyon was Sarnthein, M., Kiefer, T., Grootes, P.M., Elderfield, H., and Erlenkeuser, H., 2006, Warmings in probably higher before 4 ka than after. Higher REFERENCES CITED Aagaard, K., and Carmack, E.C., 1989, The role of the far northwestern Pacific promoted pre- biological productivity during the middle Ho- sea ice and other fresh water in the Arctic cir- Clovis immigration to America during Hein- locene is most easily explained by reduced sea culation: Journal of Geophysical Research, rich event 1: Geology, v. 34, p. 141–144, doi: ice (de Vernal et al., 2005) and a longer grow- v. 94, p. 14,485–14,498. 10.1130/G22200.1. Andrews, J.T., and Dunhill, G., 2004, Early to mid- Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., ing season, although we cannot rule out great- Hughen, K.A., Kromer, B., McCormac, G., er transport of nutrient-rich Pacific water Holocene Atlantic water influx and deglacial meltwater events, Beaufort Sea slope: Arctic van der Plicht, J., and Spurk, M., 1998, INT- through Bering Strait at that time. If middle Ocean: Quaternary Research, v. 61, p. 14–21, CAL98 radiocarbon age calibration 24,000-0 Holocene warmth reduced sea ice extent, it doi: 10.1016/j.yqres.2003.08.003. cal BP: Radiocarbon, v. 40, p. 1041–1083. Bard, E., Arnold, M., Hamelin, B., Tisnerat- Weingartner, T., Cavalieri, D., Aagard, K., and Sa- may have increased the salinity of the sea sur- saki, Y., 1998, Circulation, dense water for- face enough to maintain relatively constant Laborde, N., and Cabioch, G., 1998, Radio- carbon calibration by means of mass spectro- mation, and outflow on the northeast Chukchi ␦18 O. Salinity would have also increased as a metric 230Th/234U and 14C ages of corals: An shelf: Journal of Geophysical Research, consequence of decreased runoff to the Chuk- updated database including samples from Bar- v. 103, p. 7647–7661, doi: 10.1029/98JC0 0374. chi Sea. Alternatively, warming may have bados: Muroroa and Tahiti: Radiocarbon, v. 40, p. 1085–1092. Woodgate, R.A., and Aagaard, K., 2005, Revising been insufficient to raise sea-surface temper- the Bering Strait freshwater flux into the Arc- Christie-Blick, N., and Driscoll, N.W., 1995, Se- ␦18 tic Ocean: Geophysical Research Letters, ature enough to decrease the OofN. pach- quence stratigraphy: Annual Review of Earth v. 32, p. L02602, doi: 10.1029/2004GL02 yderma, especially if this species lives in the and Planetary Sciences, v. 23, p. 451–478. 1747. marginal ice zone or in a subsurface habitat. Cook, M.S., Keigwin, L.D., and Sancetta, C.A., Unfortunately, there are no quantitative data 2005, The deglacial history of surface and in- Manuscript received 13 February 2006 termediate water of the Bering Sea: Deep-Sea on the distribution of planktonic foraminifera Revised manuscript received 16 May 2006 Research. Part II: Topical Studies in Ocean- Manuscript accepted 18 May 2006 in Chukchi Sea waters, although zoologists ography, v. 52, p. 2163–2173, doi: 10.1016/ have observed more foraminifera in plankton j.dsr2.2005.07.004. Printed in USA

864 GEOLOGY, October 2006