Planktonic foraminifera genomic variations reflect paleoceanographic changes in the Arctic: evidence from sedimentary ancient DNA

Deciphering the evolution of marine plankton is typically based on the study of microfossil groups. Cryptic speciation is common in these groups, and large intragenomic variations occur in ribosomal RNA genes of many morphospecies. In this study, we correlated the distribution of ribosomal amplicon sequence variants (ASVs) with paleoceanographic changes by analyzing the highthroughput sequence data assigned to Neogloboquadrina pachyderma in a 140,000-year-old sediment core from the Arctic Ocean. The sedimentary ancient DNA demonstrated the occurrence of various N. pachyderma ASVs whose occurrence and dominance varied through time. Most remarkable was the striking appearance of ASV18, which was nearly absent in older sediments but became dominant during the last glacial maximum and continues to persist today. Although the molecular ecology of planktonic foraminifera is still poorly known, the analysis of their intragenomic variations through time has the potential to provide new insight into the evolution of marine biodiversity and may lead to the development of new and important paleoceanographic proxies

Novel high-pressure culture experiments on deep-sea benthic foraminifera — Evidence for methane seepage-related δ13C of Cibicides wuellerstorfi

In field studies of active hydrocarbon seeps the carbon isotopic composition of Rose Bengal stained benthic foraminiferal tests (δ13Ctest) and bottom water DIC (δ13CDIC) deviates from their normal marine ratios. This circumstance led to ongoing discussions on whether aerobic foraminifers like Cibicides wuellerstorfi are capable of living at seepage sites and, more importantly, if their tests reflect the low δ13C values of emanating methane. To evaluate the discrepancy between δ13CDIC and δ13Ctest, we conducted methane seepage-emulating culture experiments on undepressurized sediments from the Håkon Mosby Mud Volcano, a modern methane seepage structure that hosts living C. wuellerstorfi with distinct negative δ13C values. The collected sediments were cultured at a site-alike pressure and mean bottom water methane concentration using newly developed high-pressure aquaria. Over an experimental period of 5 months our novel technology enabled a successful reproduction of all calcareous deep-sea benthic foraminiferal species living at that site, notably the first C. wuellerstorfi cultured in the laboratory. To show the influence of methane on δ13Ctest, we ran parallel experiments with > 99% 12C- and 99% 13C-methane in the experimental “bottom water”. During the experimental running time methanotrophs in the water column obviously converted the experimentally added methane source to δ13C-enriched and -depleted DIC, respectively. Since whole sediment cores were cultured, it was impossible to keep δ13CDIC constant over the 5-month duration, which is reflected in a variability of δ13Ctest in foraminiferal shells. Irrespective of that, the methane source is reflected in δ13Ctest of foraminiferal shells, and for the natural seep-conditions simulating 12C-experiment the mean δ13CDIC and δ13Ctest in C. wuellerstorfi were equal. Although for future culturing experiments improvements of the experimental conditions are advisable, our first results are evidence that persistent methane emanation impacts the carbon isotopic composition of deep-sea benthic foraminifera

Geochemical evidence of a floating Arctic ice sheet and underlying freshwater in the Arctic Mediterranean in glacial periods

Numerous studies have addressed the possible existence of large floating ice sheets in the glacial Arctic Ocean from theoretical, modelling, or seafloor morphology perspectives. Here, we add evidence from the sediment record that support the existence of such freshwater ice caps in certain intervals, and we discuss their implications for possible non-linear and rapid behaviour of such a system in the high latitudes. We present sedimentary activities of 230Th together with 234U/238U ratios, the concentrations of manganese, sulphur and calcium in the context of lithological information and records of microfossils and their isotope composition. New analyses (PS51/038, PS72/396) and a re-analysis of existing marine sediment records (PS1533, PS1235, PS2185, PS2200, amongst others) in view of the naturally occurring radionuclide 230Thex and, where available, 10Be from the Arctic Ocean and the Nordic Seas reveal the widespread occurrence of intervals with a specific geochemical signature. The pattern of these parameters in a pan-Arctic view can best be explained when assuming the repeated presence of freshwater in frozen and liquid form across large parts of the Arctic Ocean and the Nordic Seas. Based on the sedimentary evidence and known environmental constraints at the time, we develop a glacial scenario that explains how these ice sheets, together with eustatic sea-level changes, may have affected the past oceanography of the Arctic Ocean in a fundamental way that must have led to a drastic and non-linear response to external forcing. This concept offers a possibility to explain and to some extent reconcile contrasting age models for the Late Pleistocene in the Arctic Ocean. Our view, if adopted, offers a coherent dating approach across the Arctic Ocean and the Nordic Seas, linked to events outside the Arctic

Living benthic foraminifers from the central Arctic Ocean

Fifty short sediment cores collected with a multiple corer and five box cores from the central Arctic Ocean were analysed to study the ecology and distribution of benthic foraminifers. To work out living faunal associations, standing stock and diversity, separate analyses of living (Rose Bengal stained) and dead foraminifers were carried out for the sediment surface. The size fractions between 63 and 125 µm and >125 µm were counted separately to allow comparison with former Arctic studies and with studies from the adjacent Norwegian-Greenland Sea, Barents Sea and the North Atlantic Ocean. Benthic foraminiferal associations are mainly controlled by the availability of food, and competition for food, while water mass characteristics, bottom current activity, substrate composition, and water depth are of minor importance. Off Spitsbergen in seasonally ice-free areas, high primary production rates are reflected by high standing stocks, high diversities, and foraminiferal associations (>125 µm) that are similar to those of the Norwegian-Greenland Sea. Generally, in seasonally ice-free areas standing stock and diversity increase with increasing food supply. In the central Arctic Ocean, the oligotrophic permanently ice-covered areas are dominated by epibenthic species. The limited food availability is reflected by very low standing stocks and low diversities. Most of these foraminiferal associations do not correspond to those of the Norwegian-Greenland Sea. The dominant associations include simple agglutinated species such as Sorosphaerae, Placopsilinellae, Komokiacea and Aschemonellae, as well as small calcareous species such as Stetsonia horvathi and Epistominella arctica. Those of the foraminiferal species that usually thrive under seasonally ice-free conditions in middle bathyal to lower bathyal water depth are found under permanently ice-covered conditions in water depths about 1000 m shallower, if present at all

Vertical distribution of benthic foraminifers in the Arctic Ocean

The vertical distribution of living (Rose Bengal stained) benthic foraminifers was determined in the upper 15 cm of sediment cores taken along transects extending from the continental shelf of Spitsbergen through the Eurasian Basin of the Arctic Ocean. Cores taken by a multiple corer were raised from 50 stations with water depths between 94 and 4427 m, from areas with moderate primary production values to areas that are among the least productive ones in the world. We believe, that in the Arctic Ocean the vertical distribution of living foraminifers is determined by the restricted availability of food. Live foraminiferal faunas are dominated by potentially infaunal species or epifaunal species. Species confined to the infaunal microhabitat are absent in Arctic sediments that we examined, and predominantly infaunal living species are nowhere dominant. In general, an infaunal mode of life is restricted to the seasonally ice-free areas and thus to areas with at least moderate primary production during the summer period. Under the permanent ice cover living species are usually restricted to the top centimeter of the sediment surface, even though some are able to dwell deeper in the sediment under ice-free conditions