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

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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 paleoenvi- ronment, 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 produc- tivity 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 ex- tent, 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 sedimenta- tion 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.
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