SPATIAL AND TEMPORAL VARIABILITY IN BENTHIC OSTRACODE ASSEMBLAGES IN THE NORTHERN BERING AND CHUKCHI SEAS, 1976 TO 2010

I examined living ostracode assemblages from the northern Bering Sea, collected between 1976 to 2010, and from the Chukchi Sea, collected in 2009 and 2010, to determine how climatic and oceanographic changes affect ostracode species distributions. I found the Bering Sea assemblage to be transitional in species composition between those inhabiting western Arctic continental shelves and the subarctic Gulf of Alaska. Temporal changes in the Bering Sea assemblage provide evidence that decadal temperature changes have affected species composition. For example, the proportion of Normanicythere leioderma, a predominantly Arctic species, decreased from 70% of the total assemblage population in 1999 to 15% by 2006. This decrease coincided with a shift in the Arctic Oscillation toward a positive mode and warmer Bering sea-surface temperatures beginning in the early 2000s. My results support the hypothesis that recent ocean temperature changes in the Bering-Chukchi Sea region are changing species composition in benthic ecosystems

Multi-proxy record of ocean-climate variability during the last two millennia on the Mackenzie Shelf, Beaufort Sea

A 2,000 year-long oceanographic history, in sub-centennial resolution, from a Canadian Beaufort Sea continental shelf site (60 meters water depth) near the Mackenzie River outlet is reconstructed from ostracode and foraminifera faunal assemblages, shell stable isotopes (δ18O, δ13C) and sediment biogenic silica. The chronology of three sediment cores making up the composite section was established using 137Cs and 210Pb dating for the most recent 150 years and combined with linear interpolation of radiocarbon dates from bivalve shells and foraminifera tests. Continuous centimeter-sampling of the multicore and high-resolution sampling of a gravity and piston core yielded a time-averaged faunal record of every ~40 years from 0 to 1850 CE and every ~24 years from 1850 to 2013 CE. Proxy records were consistent with temperature oscillations and related changes in organic carbon cycling associated with the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). Abundance changes in dominant microfossil species, such as the ostracode Paracyprideis pseudopunctillata and agglutinated foraminifers Spiroplectammina biformis and S. earlandi, are used as indicators of less saline, and possibly corrosive/turbid bottom conditions associated with the MCA (~800-1200 CE) and the most recent ~60 years (1950-2013). During these periods, pronounced fluctuations in these species suggest that prolonged seasonal sea-ice melting, changes in riverine inputs and sediment dynamics affected the benthic environment. Taxa analyzed for stable oxygen isotope composition of carbonates show the lowest δ18O values during intervals within the MCA and the highest during the late LIA, which is consistent with a 1° to 2°C cooling of bottom waters. Faunal and isotopic changes during the cooler LIA (1300-1850 CE) are most apparent at ~1500-1850 CE and are particularly pronounced during 1850 to ~1900 CE, with a ~0.5 per mil increase in δ18O values of carbonates from median values in the analyzed taxa. This very cold 50-year period suggests that enhanced summer sea ice suppressed productivity, which is indicated by low sediment biogenic silica values and lower δ13C values in analyzed species. From 1900 CE to present, declines in calcareous faunal assemblages and changes in dominant species (Cassidulina reniforme and P. pseudopunctillata) are associated with less hospitable bottom waters, indicated by a peak in agglutinated foraminifera from 1950-1990 CE

Microfaunal Recording of Recent Environmental Changes in the Herschel Basin, Western Arctic Ocean

Microfaunal assemblages of benthic foraminifera, ostracods, and tintinnids from two marine sediment cores retrieved from the Herschel Basin of the Canadian Beaufort Sea shelf document relationships with environmental parameters such as salinity, sea-ice cover, and turbulence. Cores YC18-HB-GC01 and PG2303-1 were collected at 18 and 32 m water depth, respectively. At these sites, sediment accumulation rates range between 0.6 and 1.7 cm yr–1 allowing a near-annual temporal resolution over the last 50 years. Multivariate analyses indicate that benthic foraminiferal assemblages respond primarily to food supply. Dissimilarities between the microfaunal assemblages of the two cores are mainly the result of bottom water salinity levels linked to water depth. High abundance of the benthic foraminiferal species Elphidium clavatum and occurrences of Elphidium bartletti point to varying, but relatively low, salinities at the shallow core site YC18-HB-GC01, which may be affected by variations in the summer halocline depth. Higher species diversity and more abundant Cassidulina reniforme and Stainforthia feylingi characterize the deeper core PG2303-1, which might reflect more stable conditions and higher bottom-water salinities throughout the studied time interval. The most important microfaunal shift of the last 50 years, observed in the shallower longer core YC18-HB-GC01, occurred at the turn of the 21st century. Prior to ∼2000 CE, the presence of Islandiella norcrossi indicates more stable and saline conditions. Since ∼2000 CE, increased abundances of Haynesina nivea and of the ciliate Tintinnopsis fimbriata suggest decreased salinity and increased turbidity. An increased abundance of Eoeponidella pulchella after ∼2000 CE suggests a concurrent increase in productivity in the last two decades. This shift is nearly synchronous with a decrease in mean summer sea-ice concentration, which can play an important role in bottom water stability on the shelf. Easterly winds can induce a reduction in the sea-ice cover, but also foster a westward spreading of the Mackenzie River plume and the upwelling of nutrient-rich Pacific waters onto the shelf. Both factors would explain the increased freshening and productivity of the Herschel Basin. The last two decades were also marked by a decrease in ostracod abundance that may relate to higher water turbidity. This study shows that combining information from benthic foraminifera, ostracods, and tintinnids provides a comprehensive insight into recent hydrographic/climatic changes in nearshore Arctic habitats, where productivity is critical for the food security of local communities

A 1300-year microfaunal record from the Beaufort Sea shelf indicates exceptional climate-related environmental changes over the last two centuries

<p>The environments of Arctic Ocean nearshore areas experience high intra- and inter-annual variability, making it difficult to evaluate the impact of anthropogenic warming. However, a sediment record from the southern Canadian Beaufort Sea allowed us to reconstruct the impacts of climate and environmental changes over the last 1300 years along the northern Yukon coast, Canada. The coring site (PG2303; 69.513°N, 138.895°W; water depth 32 m) is located in the Herschel Basin, where high sedimentation rates (0.1–0.5 cm a−1) allowed analyses at sub-centennial to decadal resolutions. Benthic foraminiferal, ostracod, and tintinnid assemblages, as well as the stable isotope composition of the foraminifera Elphidium clavatum and Cassidulina reniforme were used as paleoclimatic and ecological indicators, while the age model was based on the combined radiometric data of 14C, 210Pb and 137Cs. From ca 700 to 1050 CE, our data suggest penetration of offshore shelf-break waters inferred by the dominance of C. reniforme followed by the relatively abundant Triloculina trihedra in the foraminiferal assemblages as both species are associated with stable saline conditions. Afterwards, the occurrence of ostracods Kotoracythere arctoborealis and Normanicythere leioderma suggests influx of Pacific-sourced waters until ca. 1150 CE. From ∼1150–1650 CE, persistent frigid waters, limited sediment supply, and low abundances of microfossils suggest cold conditions with pervasive annual sea-ice cover that may have restricted upwelling of oceanic waters. After ∼1800 CE, the co-occurrence of Tintinnopsis fimbriata and bacterial/complex organic carbon feeder foraminifera (Quinqueloculina stalkeri, Textularia earlandi and Stetsonia horvathi), suggest an increased influence of freshwater rich in particulate organic matter, which may be related to the spreading of the Mackenzie River plume and/or increased coastal permafrost erosion during longer ice-free seasons. Based on these proxy data, the shift at ∼1800 CE marks the onset of regional warming, which further intensified after ∼1955 CE, likely in response to the anthropogenic forcing.</p&gt

Deglacial sea level history of the East Siberian Sea and Chukchi Sea margins

Deglacial (12.8–10.7 ka) sea level history on the East Siberian continental shelf and upper continental slope was reconstructed using new geophysical records and sediment cores taken during Leg 2 of the 2014 SWERUS-C3 expedition. The focus of this study is two cores from Herald Canyon, piston core SWERUS-L2-4-PC1 (4-PC1) and multicore SWERUS-L2-4-MC1 (4-MC1), and a gravity core from an East Siberian Sea transect, SWERUS-L2-20-GC1 (20-GC1). Cores 4-PC1 and 20-GC were taken at 120 and 115 m of modern water depth, respectively, only a few meters above the global last glacial maximum (LGM;  ∼  24 kiloannum or ka) minimum sea level of  ∼  125–130 meters below sea level (m b.s.l.). Using calibrated radiocarbon ages mainly on molluscs for chronology and the ecology of benthic foraminifera and ostracode species to estimate paleodepths, the data reveal a dominance of river-proximal species during the early part of the Younger Dryas event (YD, Greenland Stadial GS-1) followed by a rise in river-intermediate species in the late Younger Dryas or the early Holocene (Preboreal) period. A rapid relative sea level rise beginning at roughly 11.4 to 10.8 ka ( ∼  400 cm of core depth) is indicated by a sharp faunal change and unconformity or condensed zone of sedimentation. Regional sea level at this time was about 108 m b.s.l. at the 4-PC1 site and 102 m b.s.l. at 20-GC1. Regional sea level near the end of the YD was up to 42–47 m lower than predicted by geophysical models corrected for glacio-isostatic adjustment. This discrepancy could be explained by delayed isostatic adjustment caused by a greater volume and/or geographical extent of glacial-age land ice and/or ice shelves in the western Arctic Ocean and adjacent Siberian land areas

Multi-proxy record of ocean-climate variability during the last two millennia on the Mackenzie Shelf, Beaufort Sea

A 2,000 year-long oceanographic history, in sub-centennial resolution, from a Canadian Beaufort Sea continental shelf site (60meters water depth) near the Mackenzie River outlet is reconstructed from ostracode and foraminifera faunal assemblages, shell stable isotopes (delta 18O, delta 13C) and sediment biogenic silica. The chronology of three sediment cores making up the composite section was established using 137Cs and 210Pb dating for the most recent 150 years and combined with linear interpolation of radiocarbon dates from bivalve shells and foraminifera tests.Continuous centimeter-sampling of the multicore and high-resolution sampling of a gravity and piston core yielded a time-averaged faunal record of every approximately 40 years from 0 to 1850 CE and every approximately 24 years from 1850 to 2013 CE. Proxy records were consistent with temperature oscillations and related changes in organic carbon cycling associated with the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). Abundance changes in dominant microfossil species, such as the ostracode Paracyprideis pseudopunctillata and agglutinated foraminifers Spiroplectammina biformis and S. earlandi, are used as indicators of less saline, and possibly corrosive/turbid bottom conditions associated with the MCA (approximately 800 to 1200 CE) and the most recent approximately 60 years (1950–2013). During these periods, pronounced fluctuations in these species suggest that prolonged seasonal sea-ice melting, changes in riverine inputs and sediment dynamics affected the benthic environment. Taxa analyzed for stable oxygen isotope composition of carbonates show the lowest delta 18O values during intervals within the MCA and the highest during the late LIA, which is consistent with a 1 degree to 2 degree C cooling of bottom waters. Faunal and isotopic changes during the cooler LIA (1300 to 1850 CE) are most apparent at approximately 1500 to 1850 CE and are particularly pronounced during 1850 to approximately 1900 CE, with an approximate 0.5 per mil increase in delta 18O values of carbonates from median values in the analyzed taxa. This very cold 50-year period suggests that enhanced summer sea ice suppressed productivity,which is indicated by low sediment biogenic silica values and lower delta 13C values in analyzed species. From 1900CE to present, declines in calcareous faunal assemblages and changes in dominant species (Cassidulina reniforme and P. pseudopunctillata) are associated with less hospitable bottom waters, indicated by a peak in agglutinated foraminifera from 1950 to 1990 CE

Biogeographic, Geochemical, and Paleoceanographic Investigations of Ostracodes in the Bering, Chukchi, and Beaufort Seas

In this study, I investigated the continental shelf environments of the Bering, Chukchi, and Beaufort Seas using species of Ostracoda and their shell chemistry as indicators of oceanographic conditions and change. Ostracodes are bivalved Crustacea that secrete a calcareous shell commonly preserved in sediments in the Arctic. Because ostracode species have survival limits controlled by temperature, salinity, oxygen, sea ice, food, and other habitat-related factors, they are useful ecological indicators. A primary objective of my dissertation research was to establish how their ecology, biogeography and shell geochemistry is related to ocean variability in water mass properties and productivity at high latitudes. First, I examined community assemblages of ostracodes over several decades (1970-2018) in the northern Bering, Chukchi, and Beaufort Seas, and the main environmental factors that affect their biogeography. Results showed that large-scale south-to-north and small-scale nearshore-offshore gradients in ostracode community structure were tied to changes in water mass properties in combination with food sources and sediment substrate. Although the dominant species did not significantly change over the investigated period, the frequency of two cold-temperate species that are primarily and previously restricted to shallow North Pacific sediments off Asia has increased during the last decade. This suggests that these species are responding to recent increases in coastal and mid-shelf bottom water temperatures and/or carbon flux to the benthos. A second goal was to assess the feasibility of using stable oxygen isotopes (δ18O) of carbonate from ostracode shells as paleoceanographic proxies for water mass identification on Arctic and subarctic continental shelves. Through the use of regression analyses, I established that the δ18O values of carbonates from two species (of five investigated) can be reliable recorders of summer water mass changes in temperature and seawater δ18O content. The third part of the study was to use results from these prior two goals in combination with data on biogenic silica, foraminifera assemblages and stable isotope composition of biogenic carbonates, to reconstruct 2,000 years of paleoceanography from a radiocarbon-dated sediment core on the Mackenzie Shelf of the Beaufort Sea. This high-resolution (sub-centennial) record identified shifts in multiple proxies that are related to climate oscillations such as the Medieval Climate Anomaly, the Little Ice Age, and the modern period of anthropogenic change. The overall findings of my dissertation research support the premise that on complex and dynamic continental shelves, paleoceanographic uncertainties can be addressed by documenting microfossil faunal assemblages, measuring stable isotope variability in microfossil carbonates, as well as relating the distribution of species in time with an understanding of species ecology