Late Quaternary Paleoceanography Research Articles

Late Quaternary paleoceanography of Vestnesa Ridge, Fram Strait: Ostracode species as a potential indicator of cold seep activity

Abstract Past intensity of methane release from deep-ocean methane hydrates continues to be challenging to reconstruct reliably. Here, we used fossil ostracode fauna paired with foraminiferal δ13C values in a marine sediment core from Vestnesa Ridge, western Svalbard margin, to reconstruct methane seepage activity during the late Quaternary and to examine faunal response to deglacial climatic changes. Benthic foraminiferal δ13C values indicate methane seepage activity was relatively strong during marine isotope stage 2, corresponding to a high percentage of the ostracode Rosaliella svalbardensis in the assemblage. In contrast, this species was absent under conditions of no or very strong seepage of methane. Faunal changes in other taxa were more related to global climate changes regardless of the seepage activity. This result indicates that Rosaliella svalbardensis is a potential new useful proxy for past methane release.

Sediment provenance and paleoenvironmental changes in the northwestern shelf mud area of the South China Sea since the mid-Holocene

The late Quaternary paleoceanography and paleoenvironment of the South China Sea (SCS) have been well reconstructed over the last decade. In contrast, the provenance of the terrigenous sediments that have accumulated in the northwestern continental shelf mud area remains enigmatic. This study investigated the provenance of these sediments and the paleoenvironmental changes archived in Core X2 via the analysis of geochemical elements, grain size, and accelerator mass spectrometry (AMS) 14C ages. Based on the upper continental crust (UCC)-normalized REE patterns and REE fractionation parameters, southwestern and western Taiwanese rivers and the Pearl River were identified as the main sources of the fine-grained sediment deposited in the northwestern shelf mud area off Hainan Island. This finding further confirms the long-distance transport (> 1000km) of fine-grained sediment from Taiwanese rivers to the northern SCS shelf and slope. Obvious changes in the grain size and Chemical Index of Alteration (CIA) record occurred at approximately 4.0cal kyr BP and were likely caused by increased Hainan Island inputs due to sea level changes.

Provenance discrimination of siliciclastic sediments in the middle Okinawa Trough since 30 ka: Constraints from rare earth element compositions

The late Quaternary paleoceanography and paleoenvironment in the Okinawa Trough, East China Sea, have been well reconstructed over the last decade, while in contrast the provenance of terrigenous sediments that have accumulated there remains enigmatic. In this study, rare earth elements (REE) were used to investigate provenance changes in sediments from Core DGKS9604, taken from the middle Okinawa Trough. Discrimination plots based on REE fractionation parameters suggest that the cored sediments have variable provenances over the last 30 ka, with the lower part (ca. 31–8.2 ka) ultimately originating mostly from the Changjiang (Yangtze River) and the upper part (7.1–0 ka) primarily from Taiwan. During the Last Glacial Maximum and early deglacial period, sea level was low and the main stream of the Kuroshio Current was deflected to the east of the Ryukyu Islands. As a result the Changjiang-derived sediments might have dominated sedimentation of the middle Okinawa Trough. However, since about 7 ka the main stream of the Kuroshio Current strengthened in the area of the trough, as sea level approximated the modern position. This caused near-bottom transport of fine-grained sediments from the continental margin to the trough to become weak and instead, Taiwan-derived terrigenous sediments dominated in the middle trough. The changing provenances of terrigenous sediments into the middle Okinawa Trough are closely related to the evolution of oceanic circulation and sea level in the East China Sea. Two tephra layers in the core have distinct REE compositions and correlate well with two volcanic eruptions at 7.6 and 25.8 ka in southern Japan.

Late Quaternary paleoceanography and paleo-sea ice conditions in the Mackenzie Trough and Canyon, Beaufort SeaThis article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.

The Mackenzie Trough provides a high resolution signal for paleoceanography as a result of high sedimentation rates at the mouth of the Mackenzie River. Three cores were collected along a transect covering a depth range of 58–671 m and the time period of the last 11 500 cal BP. Prior to the last ∼10 000 cal BP, the distal core is characterized by laminated sediment and a foraminiferal fauna of Arctic Bottom Water calcareous species and abundant planktic foraminifera suggesting little freshwater runoff and (or) perennial sea-ice cover. This occurs at a similar time as laminated sediments from the west of this site, which have been suggested to be part of the Lake Agassiz flood outburst and (or) cold period. If this outburst occurred, the very positive oxygen isotope values from our core (PC3; >+3.0 ppm) indicate that it did not flow through the Mackenzie Trough. After 9000 cal BP, the faunas in the three cores differ because of timing and different water depths. However, it is possible to see a progression of cold saline water prior to 10 000 cal BP, with a freshening of surface water after 10 000 cal BP where tintinnids (brackish water ciliates) occur with incursions of deep water Arctic calcareous species to ∼3000 years BP. A sequence of mixed faunas appears as sea ice returns, at least periodically in the last 3000 cal BP; but (in core PC2 only) a return to more sea ice is recorded by both foraminifera and dinocysts in the last few hundred years.

Introduction to the Special Section “Proxy Records and Mechanistic Studies of the Late Quaternary Paleoceanography of the Western Pacific Marginal Seas Using IMAGES Cores”

The western Pacific Ocean plays an important role in regulating and modulating the global climate system on interannual to geological timescales. Excellent records have been acquired by coring high sedimentation rate sites in the marginal seas of the western Pacific Ocean during IMAGES cruises. This special section has been proposed to publish papers presenting high-resolution records and the interpretation of climate change mechanisms revealed by these records. Chen et al. (2008) established a long and detailed chro

Late Quaternary Paleoceanography and Sedimentary Environments in Hudson Strait

Airgun and high resolution Huntec seismic reflection profiles are interpreted to show up to 130 m of glacial, glaciomarine and postglacial sediments overlying bedrock. In a basin at the eastern entrance to Hudson Strait most of the surficial sediment was deposited during the last déglaciation, but in western Hudson Strait multiple till sequences from previous glaciations are recognized. Five acoustic units were identified, at least three of which were penetrated with piston cores. Foraminifera of the stratigraphically deepest core in the eastern basin indicate a proximal glaciomarine environment and a likely presence of an ice shelf. A 14C date of 8060 ± 70 yBP (TO 750) on molluscan shells gives a minimum age for the top of the acoustically laminated distal glaciomarine sediments. The early postglacial foraminifera suggest a period of increased influence of offshore bottom waters restricted to the deep eastern basin. The surface sediments of all cores contain species indigenous of colder and fresher inshore waters of the present time. The ratio of 18CV16O in the benthic foraminifer Cibicides lobatulus is herein related to bottom salinity. Downcore measurements of 8'8O on C. lobatulus tests indicate bottom paleosalinities lower by about 0.5%o shortly before the dated horizon of 8000 yBP. By this time Hudson Strait was sufficiently clear of glacial ice for establishment of the present tidal regime. The lower bottom salinities indicate that tidal mixing took place between glacial meltwater leaving Hudson Bay and the offshore counterflow. This process is thought to have reduced the sharpness of the salinity difference between the offshore water and the surface plume of Laurentide meltwater as it entered the ocean.

Late Quaternary paleoceanography deduced from coccolith assemblages in a piston core recovered off the central Japan coast

Nannofossil assemblages from the last 144 ka (∼Marine Isotope Stage, MIS6) were studied in a piston core (MD01-2421) from the NW Pacific (36°01.4′N, 141°46.8′E, WD: 2,224 m), off the coast of central Japan. The site is influenced by both the subtropical Kuroshio and the subarctic Oyashio Currents, and is located just north of the Kuroshio Front. This study attempted to reconstruct paleoceanographic changes in the region by illustrating the past migration patterns of the Kuroshio Current, which is considered to be one of the strongest western boundary currents in the Pacific. In order to reconstruct the past migration patterns of the Kuroshio Current, relative abundance values of several taxa classified as either “Kuroshio Species” or “Oyashio Species” were used as indices. Using these species, a ratio (Tn: Nannofossils temperature index), representing the mixing of the Kuroshio and Oyashio Currents, was calculated to express quantitatively the influence of the Kuroshio Current. Tn values fluctuated widely, ranging from 1.45 (135 ka) to 86.9 (115.3 ka), and corresponds to the warm–cold cycles obtained by oxygen isotope analysis. The fluctuation pattern of percentage abundance of Umbellosphaera spp. shows a strong similarity to that of the precessional cycle. It may indicate that subtropical water has influenced the Japanese coast during these warm intervals, possibly due to the expansion of subtropical divergence. Also, the fluctuation pattern of Tn shows a similar time-transgressive trend to that of the precessional cycle. This implies that latitudinal displacement of the Kuroshio Front was related to subtropical climatic variability. The ratio between Tn and Tp (planktonic foraminifera temperature index) drastically increased at about 20 ka, in MIS2, with small peaks in MIS6. This suggests that a thin layer of warm water may have covered the study area during these intervals. An increased preservation of organic matter (i.e. CaCO 3 preservation) possibly affected the absolute coccolith abundance (coccolith/g dry sediment), although other factors controlling absolute coccolith abundance (e.g. nutrient levels) need to be taken into consideration.

Paleoceanographic implications of a 90,000 year long diatom record in piston core KH94-3, LM-8 off NE Japan

Q-mode principal component analysis (PCA) of a fossil diatom assemblage reveals the late Quaternary paleoceanography of the NW Pacific margin, based on submarine core KH94-3, LM-8 recovered off the Sanriku coast of NE Japan (38°53.46′N, 143°22.19′E). Overall, diatoms were abundant during interglacials, but low in abundance and poor in preservation during the last glacial, with each oxygen isotope stage having its own characteristic assemblage. The results of PCA are: principal component 1 (PC1) is composed mainly of Thalassionema nitzschioides, which predominated during the coldest oxygen isotopic Stages 4 (75–58 ka) and 2 (25–11 ka), and by allochthonous taxa. The diatom tests are poorly preserved and the diatom mass accumulation rate (DMAR) is the lowest in these stages. PC2 is characterized by ice-related diatoms that predominate in Stage 3 (the interstadial), whereas PC3, which is characterized by Neodenticula seminae and Odontella aurita, shows high values in Stage 1. Diatom assemblages exhibit drastic changes between oxygen isotopic Stages 3 and 1 (11–0 ka). Distinct increases of DMAR occurred during both periods. We infer an intensified advection of sea ice from the Sea of Okhotsk toward the area off Sanriku during Stage 3. This interpretation explains the high DMAR in Stage 3, owing to high productivity at the sea ice edge. Our study can contribute to marine environmental reconstruction since the last glaciation because fewer micropaleontological data exist in the NW Pacific margin for these time intervals, compared with other oceans.

Late Quaternary Paleoceanography of the Japan Sea: An Update.

Because the Japan Sea is a semi-enclosed marginal sea surrounded by a continent and an island arc, its oceanographic condition during late Quaternary has been influenced by many factors including climate in central to east Asia and Japan, oceanographic condition of northwestern Pacific, and glacioeustatic sea level changes. Consequently, its sedimentary record is complicated and difficult to interpret. However, the record preserves a wealth of information both terrestrial and marine, and is suited for linking the terrestrial climate in central to east Asia and the oceanography of the Japan Sea and surrounding oceans. Recent progress revealed intimate linkage between the Asian climate and the Japan Sea paleoceanography on millennial time-scale.

PALEOCEANOGRAPHY OF THE LAST INTERGLACIAL/GLACIAL CYCLE IN THE POLAR NORTH ATLANTIC

Using to its position close to the sites of waxing and waning of the major northern hemisphere ice sheets during the Late Quaternary climatic cycles, the Polar North Atlantic plays a key role in driving global change. Therefore, analyzing the present-day sedimentation processes and reconstructing the Late Quaternary paleoceanography in this region has been a focus of major scientific interest through the last fifteen years. This paper aims to combine the vast amount of new results about modern sediment fluxes, surface sediment distribution and the paleoceanographic record and to present a comprehensive overview of the paleoceanography in the Polar North Atlantic through the last 200,000 years, which is based on the basinwide analysis of carbonate and ice-rafted detritus records. During the last fifteen years the CLIMAP paleoceanographic view of the glacial Polar North Atlantic as an almost permanently isolated sea covered by heavy sea ice throughout the year has been shifted to a much more dynamic view of the environmental conditions. A meridional current system similar to the present-day conditions secured the exchange of water, ice and heat between the Polar North Atlantic and the Alantic and Arctic Oceans. Although affected by variations in strength and intensity this meridional current pattern resulted in an almost permanent presence of at least some seasonally ice-free areas, with all the consequences for e.g. marine life and deep water formation. In addition, the development of the continental ice sheets, namely the Fennoscandian and the Barents Sea ice sheets, is closely related to this dynamic circulation pattern. During the so-called Nordway events the pronounced inflow of temperate waters from the south provided moisture for the growth of the ice sheets. The most prominent of these events (Stage 6, Stage 4 and Substage 3.1/2) ended in major glaciations, reflected in the terrestrial sequences and in the deep-sea IRD records.

Late Quaternary Paleoceanography of the North Atlantic Margins

Late Quaternary Paleoceanography of the North Atlantic Margins

Late Quaternary paleoceanography in the Fram Strait

Four sediment cores from the Fram Strait (78°N) have been studied to reconstruct the paleoceanography of this major connection of the Arctic Ocean to the world ocean. Back to oxygen isotope stage 5 (128 kyr ago) the stratigraphic interpretation of the cores is based on oxygen isotope and accelerator mass spectrometry data; the identification of oxygen isotope stage 6 is based on paleomagnetic and coccolith data. Parameters indicative for contrasting environmental conditions such as high input of ice‐rafted detritus (IRD) or seasonally open waters display characteristic sequences by which the paleoce anography of this region can be reconstructed for the last 180 kyr. The geographic setting of Fram Strait and the sediment data indicate a permanent meridional circulation pattern throughout the last 180 kyr, as it is typical, for example, for the present‐day circulation. Depending on the strength of advection, Atlantic waters appeared in Fram Strait as surface waters, resulting in seasonally ice‐free conditions, or as subsurface water masses, underlying polar waters perenially covered by ice. Sustained periods of seasonally ice‐free waters were largely restricted to interglacial stages 5.5, 5.1, and the Holocene. However, a few short events characterized by seasonally ice‐free conditions at 78°N are recorded during glacial stages as well, for example, stage 6, late stage 3, and stage 2 (“Nordway” events). These events provide for supply of moisture, and therefore they are closely related to the history of the European Arctic ice sheets. Strong input of IRD, reflecting severe glaciations on surrounding landmasses, occurred during most of stage 6, in early stage 3, and during stage 2. Variations in IRD input allow differentiation between three major source areas: (1) Svalbard/Barents Sea during major glacier advances in stages 2 to 5; (2) most likely Siberia during stage 6; and (3) Fennoscandia during the short events in glacial periods marked by seasonal open waters. Thus the circulation pattern in the Fram Strait remained rather stable throughout the last 180 kyr, while variations in the sediment composition are mainly due to the intensity of Atlantic water advection and to the prevailing IRD input pathway.

Late Quaternary Paleoceanography of the Mid- to Outer Continental Shelf, East Greenland

Late Quaternary Paleoceanography of the Mid- to Outer Continental Shelf, East Greenland

Late Quaternary paleoceanography of the South China Sea: surface circulation and carbonate cycles

Paleoceanographic information from 34 sediment cores is summarized to investigate the glacial-interglacial variations in sea surface circulation and late Quaternary carbonate cycles in the South China Sea. Judging from the distribution pattern of deposition rates, the enormous terrigenous supply by rivers is responsible for the high rate of hemipelagic sedimentation which was even higher during glacial periods. Paleotemperature maps based on Transfer Function temperature estimates have revealed a E-W gradient of SST for glacial summer and a S-N gradient for glacial winter, and this agrees well with the clockwise surface circulation in summer and counterclockwise in winter during glacials, as shown by a preliminary computer modelling. The high-resolution paleotemperature curves of core V36-3 from the northern slope show an abrupt cooling event which may belong to the Younger Dryas. Carbonate percentage in sediments and preservation of foraminifers and pteropods are used to study the carbonate cycles in the marginal sea. It turns out that two mechanisms are functioning there. Being a basin with large drainage areas, vast amount of river-borne terrigenous input dilutes carbonate, and its intensification during glacial decreases the glacial %CaC0 3; being a basin connected with the Pacific, the fluctuations of lysocline, CCD and ACD in the SCS are related to those in the Pacific Ocean, and dissolution is strengthened at interglacial. Thus, above the present CCD the dilution factor predominates and the “Atlantic cycles” are produced; below the present CCD the dissolution factor prevails, resulting in the “Pacific” pattern of carbonate cycles.

Late Quaternary paleoceanography of the Eurasian Basin, Arctic Ocean

We reconstructed late Quaternary deep (3000–4100 m) and intermediate depth (1000–2500 m) paleoceanographic history of the Eurasian Basin, Arctic Ocean from ostracode assemblages in cores from the Lomonosov Ridge, Gakkel Ridge, Yermak Plateau, Morris Jesup Rise, and Amundsen and Makarov Basins obtained during the 1991 Polarstern cruise. Modern assemblages on ridges and plateaus between 1000 and 1500 m are characterized by abundant, relatively species‐rich benthic ostracode assemblages, in part, reflecting the influence of high organic productivity and inflowing Atlantic water. In contrast, deep Arctic Eurasian basin assemblages have low abundance and low diversity and are dominated by Krithe and Cytheropteron reflecting faunal exchange with the Greenland Sea via the Fram Strait. Major faunal changes occurred in the Arctic during the last glacial/interglacial transition and the Holocene. Low‐abundance, low‐diversity assemblages from the Lomonosov and Gakkel Ridges in the Eurasian Basin from the last glacial period have modern analogs in cold, low‐salinity, low‐nutrient Greenland Sea deep water; glacial assemblages from the deep Nansen and Amundsen Basins have modern analogs in the deep Canada Basin. During Termination 1 at intermediate depths, diversity and abundance increased coincident with increased biogenic sediment, reflecting increased organic productivity, reduced sea‐ice, and enhanced inflowing North Atlantic water. During deglaciation deep Nansen Basin assemblages were similar to those living today in the deep Greenland Sea, perhaps reflecting deepwater exchange via the Fram Strait. In the central Arctic, early Holocene faunas indicate weaker North Atlantic water inflow at middepths immediately following Termination 1, about 8500–7000 year B.P., followed by a period of strong Canada Basin water overflow across the Lomonosov Ridge into the Morris Jesup Rise area and central Arctic Ocean. Modern perennial sea‐ice cover evolved over the last 4000–5000 years. Late Quaternary faunal changes reflect benthic habitat changes most likely caused by changes in the import of cold, deepwater of Greenland Sea origin and warmer and middepth Atlantic water to the Eurasian Basin through the Fram Strait, and export of Arctic Ocean deepwater.Appendix Tables Al‐A4 are available on microfiche. Orderfrom the American Geophysical Union, 2000 Florida Avenue,N.W., Washington, DC 20009. Document P94‐002; $2.50.Payment must accompany order.

Three-dimensional numerical modeling of Late Quaternary paleoceanography and sedimentation in the northern North Atlantic

Modelling of Late Quaternary paleoceanography and sedimentation in the northern North Atlantic (NNA) is achieved by coupling the ocean general circulation model SCINNA (Sensitivity and Circulation In the NNA) to the sedimentation models SENNA (Sedimentation In the NNA) and PATRINNA (PArticle Tracing In the NNA).

Late quaternary paleoceanography of the Banda Sea

A stable oxygen and carbon isotopic record of benthic ( Uvigerina and Cibicidoides species) and planktonic ( Globigerinoides ruber) foraminifera recovered from a deep-sea core in the Banda Sea was obtained to develop a better understanding of surface- and deep water hydrography in this region over the last ~180 kyr. Higher glacial to interglacial δ 18O amplitudes of planktonic and benthic foraminifera (1.8 and 1.7%., respectively) indicate an increase in Banda surface and deep water salinity during glacial conditions. Planktonic data is influenced by the precession (23 kyr periodicity) while benthic values reflect intermediate Pacific water fluctuations. The benthic δ 13C exhibits a depletion of about 0.3%. for the last glacial maximum (LGM) relative to the Holocene. A still higher depletion of benthic δ 13C (0.4–0.6%.) occurs during the penultimate full glaciation (stage 6). The main characteristics of δ 13C in the Banda Sea are a direct record of the δ 13C global oceanic signal and an evolution similar to intermediate and deep Pacific waters. Banda Sea records indicate a general good ventilation that remained unchanged between glacial and interglacial periods. Variations in the carbon isotopic composition of planktonic foraminifera are attributed to the local variability in monsoonal upwelling or changes in throughflow of Pacific intermediate-water masses via Indonesian seas. An increase in the alkalinity and deepening of the lysocline has resulted in higher carbonate content during glacial periods, similar to the results reported in the North Pacific.

Late Quaternary paleoceanography of deep and intermediate water masses off Gaspé Peninsula, Gulf of St. Lawrence: foraminiferal evidence

Radiocarbon-dated benthonic foraminiferal zones in three cores provide new information on the evolution of the deep and intermediate water masses off Gaspé Peninsula. The deglacial phase in the deep Laurentian Channel began before 14 000 BP and was characterized by low-salinity (<20‰) or alternating low-salinity and saline (~35‰) water. This was followed by a cold saline phase, which ended ca. 13 500 BP, and a salinity minimum (30–33.5‰), which began ca. 12 100 BP. Between 8700 and 7900 BP, the temperature and salinity of the deep water mass increased, resulting in the modern deep water mass (temperature 4–6 °C, salinity 34.5–34.9‰) at the end of the Goldthwait Sea episode. The salinity of the deep water was apparently controlled by the meltwater flux from the ice front during the deglacial phase. After the deglacial phase the characteristics of the deep water mass were determined by the composition of offshore water entering the Laurentian Channel. Runoff from the Lake Agassiz – Great Lakes system does not appear to have mixed with the deep water of the Goldthwait Sea. The deglacial phase in Chaleur Trough, which is within the intermediate water mass, began before 12 200 BP. The temperature of the intermediate water mass has remained close to 0 °C after deglaciation; however, the salinity has increased from 25–30‰ at 12 200 BP to about 33.5‰ by 5900 BP.

Late Quaternary benthic foraminiferal biofacies and paleoceanography of Queen Charlotte Sound and southern Hecate Strait, British Columbia

Analysis of Late Quaternary benthic foraminifera in cores from Queen Charlotte Sound and Hecate Strait yields new information on the paleoceanographic history of the region. Q-mode cluster analysis grouped samples into seven associations reflecting benthic foraminiferal biofacies: the Gyroidina-Bolivina Biofacies and Gyroid­ ina-Seabrookia Biofacies are relict lower to middle bathyal faunas introduced to shallower depths when cooler water masses influenced the area; the Cribroel­ phidium Biofacies, I slandiella Biofacies, Epistominella Biofacies, and Bucella Biofacies are similar to each other in faunal makeup and characterize varying depositional conditions at neritic depths; the Lobatuln-Gavelinopsis Biofacies characterizes shallow, high energy, shelly banks. Repetition of many of these biofacies in the cores indicates an overall instability, or cyclic nature, of the water masses affecting the region through the Late Qua­ ternary. A gradual increase in the proportion of Buliminelln elegantissima, a foraminifer associated with terriginous plant debris suggests a gradual decrease in sea level dur­ ing deposition of the Bl lithologic unit (~= 12,000 years BP in the cores presented here) deposited at most sites. A population maximum of Buliminella elegantissima in diachronous lag deposits of the B2 lithologic unit (=11,600-10,000 years BP) indicates a maximum low­ stand. The almost complete absence of this species in superjacent B3 lithologic unit sediments indicates subse­ quent rapid subsidence and termination of terrestrial organic input, probably due to the collapse of the Fraser Glaciation forebulge. Deposition of the B3 unit had ter­ minated everywhere in the region by =9,000 years BP. A Cassidulina reniforme population maximum occurs in sediments dated between about 10,100 and 11,300 years B.P. The presence of high proportions of this species, associated with very cold water (glacial or near-glacial) regimes, indicates a brief interval of cooler conditions during deglaciation, isochronous with the Younger Dryas event of Europe and eastern North America. This suggests that this event is not restricted to the North Atlantic but represents a much more widespread phe­ nomenon.

Late Quaternary paleoceanography of the western Baffin Bay region: evidence from fossil diatoms

Seven marine cores from the east coast of Baffin Island were examined for variations in diatom content. The cores have good age control and were used for Holocene paleoceanographic reconstruction. A diatom barren zone or a zone of much reduced diatom productivity is evident some time during the early to middle Holocene. This zone ends earlier in the southern area than farther north. The changes in diatom productivity may be caused by changing oceanographic variables such as sea ice extent and the presence of a meltwater cap during deglaciation. Light isotope 18O events ("meltwater spikes") generally coincide with barren or reduced productivity zones in the cores from the middle and southern part of the shelf and from Jones Sound, suggesting a general surficial cooling of the fjord and ocean water. If diatom productivity was depressed because of a meltwater cap over the core sites with increasing sea ice extent, the theory of a general "marine optimum" in all of Baffin Bay and Davis Strait from 8000 to 6000 BP may have to be modified.