Consistent Changes in Nitrate Consumption in Two Sectors of the Antarctic Zone Through the Last Glacial Cycle

We present new diatom-bound N isotope (δ15Ndb) records and complementary data from two sediment cores in the Indian sector of the Southern Ocean, with age models that benefit from correlation of TEX86 to Antarctic ice-core temperature. An unprecedented degree of consistency is observed between these two δ15Ndb records and with a previously published δ15Ndb record from the Pacific sector of the Southern Ocean. Even modest δ15Ndb excursions within the progression toward the LGM (e.g. one centered on MIS 5a at ~85 ka) are found to apply in both the Indian and Pacific sectors of the Antarctic. That such similar changes in δ15Ndb are measured on nearly opposite sides of the Antarctic continent argues that they represent the overall history of δ15Ndb in the open Antarctic Zone. In general, across the last glacial cycle, δ15Ndb is found to be strongly correlated with Antarctic climate and atmospheric CO2 ,with higher δ15Ndb (and thus the inference of more complete nitrate consumption) during Antarctic cold intervals. The low opal and biogenic barium export previously documented for the glacial Antarctic Zone is observed in the Indian sector cores as well. Thus, the data comport with the interpretation of reduced nitrate supply during glacial periods. In one Indian sector core with appropriate sedimentation rate, sampling density and age constraints, minima in δ15Ndb are observed during Marine Isotope Stage (MIS) 3 that appear to reflect the Antarctic Isotope Maxima (warm) events, indicating that the same processes producing glacial/interglacial changes in nitrate supply also operate at smaller amplitude and on shorter timescales against the glacial background state. We suggest that this and other findings from the data are best explained by a westerly wind-driven mechanism for the glacial/interglacial changes in nitrate supply. Finally, there are characteristic δ15Ndb histories shared by all records during the Holocene and previous interglacial MIS 5e, which have implications for the climate/CO2 relationships that have been reconstructed for these interglacials.info:eu-repo/semantics/publishe

Southern Ocean upwelling, Earth’s obliquity, and late Pleistocene glacial-interglacial atmospheric CO2 change

info:eu-repo/semantics/nonPublishe

Data-based filtering for replicated high-throughput transcriptome sequencing experiments

Motivation: RNA sequencing is now widely performed to study differential expression among experimental conditions. As tests are performed on a large number of genes, stringent false-discovery rate control is required at the expense of detection power. Ad hoc filtering techniques are regularly used to moderate this correction by removing genes with low signal, with little attention paid to their impact on downstream analyses. Results: We propose a data-driven method based on the Jaccard similarity index to calculate a filtering threshold for replicated RNA sequencing data. In comparisons with alternative data filters regularly used in practice, we demonstrate the effectiveness of our proposed method to correctly filter lowly expressed genes, leading to increased detection power for moderately to highly expressed genes. Interestingly, this data-driven threshold varies among experiments, highlighting the interest of the method proposed here. Availability: The proposed filtering method is implemented in the R package HTSFilter available on Bioconductor. Contact: [email protected] Supplementary information: Supplementary data are available at Bioinformatics online

Bioactive trace metals and their isotopes as paleoproductivity proxies: An assessment using GEOTRACES-era data

International audiencePhytoplankton productivity and export sequester climatically significant quantities of atmospheric carbon dioxide as particulate organic carbon through a suite of processes termed the biological pump. How the biological pump operated in the past is therefore important for understanding past atmospheric carbon pump requires proxies. Due to their intimate association with biological processes, several bioactive trace metals and their isotopes are potential proxies for past phytoplankton productivity, including: iron, zinc, copper, cadmium, molybdenum, barium, nickel, chromium, and silver. Here we review the oceanic distributions, driving processes, and depositional archives for these nine metals and their isotopes based on GEOTRACES-era datasets. We offer an assessment of the overall maturity of each isotope system to serve as a proxy for diagnosing aspects of past ocean productivity and identify priorities for future research. This assessment reveals that cadmium, barium, nickel, and chromium isotopes offer the most promise as tracers of paleoproductivity, whereas iron, zinc, copper, and molybdenum do not. Too little is known about silver to make a confident determination. Intriguingly, the elements that are least sensitive to productivity may be used to trace other aspects of ocean chemistry, such as nutrient sources, particle scavenging, organic complexation, and ocean redox state. These complementary sensitivities suggest new opportunities for combining perspectives from multiple proxies that will ultimately enable painting a more complete picture of marine paleoproductivity, biogeochemical cycles, and Earth's climate history