Search for: All records

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.

 

Ostracoda (bivalved Crustacea) comprise a significant part of the benthic meiofauna in the Pacific-Arctic region, including more than 50 species, many with identifiable ecological tolerances. These species hold potential as useful indicators of past and future ecosystem changes. In this study, we examined benthic ostracodes from nearly 300 surface sediment samples, >34,000 specimens, from three regions—the northern Bering, Chukchi and Beaufort Seas—to establish species’ ecology and distribution. Samples were collected during various sampling programs from 1970 through 2018 on the continental shelves at 20 to ~100m water depth. Ordination analyses using species’ relative frequencies identified six species, Normanicythere leioderma , Sarsicytheridea bradii , Paracyprideis pseudopunctillata , Semicytherura complanata , Schizocythere ikeyai , and Munseyella mananensis , as having diagnostic habitat ranges in bottom water temperatures, salinities, sediment substrates and/or food sources. Species relative abundances and distributions can be used to infer past bottom environmental conditions in sediment archives for paleo-reconstructions and to characterize potential changes in Pacific-Arctic ecosystems in future sampling studies. Statistical analyses further showed ostracode assemblages grouped by the summer water masses influencing the area. Offshore-to-nearshore transects of samples across different water masses showed that complex water mass characteristics, such as bottom temperature, productivity, as well as sediment texture, influenced the relative frequencies of ostracode species over small spatial scales. On the larger biogeographic scale, synoptic ordination analyses showed dominant species— N . leioderma (Bering Sea), P . pseudopunctillata (offshore Chukchi and Beaufort Seas), and S . bradii (all regions)—remained fairly constant over recent decades. However, during 2013–2018, northern Pacific species M . mananensis and S . ikeyai increased in abundance by small but significant proportions in the Chukchi Sea region compared to earlier years. It is yet unclear if these assemblage changes signify a meiofaunal response to changing water mass properties and if this trend will continue in the future. Our new ecological data on ostracode species and biogeography suggest these hypotheses can be tested with future benthic monitoring efforts. 

Abstract. The northern sector of the Greenland Ice Sheet is considered to beparticularly susceptible to ice mass loss arising from increased glacierdischarge in the coming decades. However, the past extent and dynamics ofoutlet glaciers in this region, and hence their vulnerability to climatechange, are poorly documented. In the summer of 2019, the Swedish icebreakerOden entered the previously unchartered waters of Sherard Osborn Fjord, whereRyder Glacier drains approximately 2 % of Greenland's ice sheet into theLincoln Sea. Here we reconstruct the Holocene dynamics of Ryder Glacier andits ice tongue by combining radiocarbon dating with sedimentary faciesanalyses along a 45 km transect of marine sediment cores collected betweenthe modern ice tongue margin and the mouth of the fjord. The resultsillustrate that Ryder Glacier retreated from a grounded position at thefjord mouth during the Early Holocene (> 10.7±0.4 ka cal BP) and receded more than 120 km to the end of Sherard Osborn Fjord by theMiddle Holocene (6.3±0.3 ka cal BP), likely becoming completelyland-based. A re-advance of Ryder Glacier occurred in the Late Holocene,becoming marine-based around 3.9±0.4 ka cal BP. An ice tongue,similar in extent to its current position was established in the LateHolocene (between 3.6±0.4 and 2.9±0.4 ka cal BP) andextended to its maximum historical position near the fjord mouth around 0.9±0.3 ka cal BP. Laminated, clast-poor sediments were deposited duringthe entire retreat and regrowth phases, suggesting the persistence of an icetongue that only collapsed when the glacier retreated behind a prominenttopographic high at the landward end of the fjord. Sherard Osborn Fjordnarrows inland, is constrained by steep-sided cliffs, contains a number ofbathymetric pinning points that also shield the modern ice tongue andgrounding zone from warm Atlantic waters, and has a shallowing inlandsub-ice topography. These features are conducive to glacier stability andcan explain the persistence of Ryder's ice tongue while the glacier remainedmarine-based. However, the physiography of the fjord did not halt thedramatic retreat of Ryder Glacier under the relatively mild changes inclimate forcing during the Holocene. Presently, Ryder Glacier is groundedmore than 40 km seaward of its inferred position during the Middle Holocene,highlighting the potential for substantial retreat in response to ongoingclimate change.