Elevated Post K-Pg Export Productivity in the Gulf of Mexico and Caribbean

The global heterogeneity in export productivity after the Cretaceous-Paleogene (K-Pg) mass extinction is well documented, with some sites showing no change on geologic timescales, some demonstrating sustained decline, and a few showing a somewhat surprising increase. However, these records come from sites so widespread that a key outstanding question is the geographic scale of changes in export productivity, and whether similar environments (open ocean gyres, western boundary currents) responded similarly or whether heterogeneity is unrelated to environment. To address this, we developed three new Ba/Ti export productivity records from sites in the Gulf of Mexico and Caribbean which, combined with published data from a fourth site in the Chicxulub Crater itself, allows us to reconstruct regional changes in post K-Pg export productivity for the first time. We find that, on a regional scale, export productivity change is homogenous, with all four sites showing a ~300 kyr period of elevated export production just after the boundary, followed by a longer period of decline. Interestingly, this interval of elevated export production appears to coincide with the post K-Pg global micrite layer, which is thought to at least partially have been produced by blooms of carbonate-producing cyanobacteria and other picophytoplankton. We note from a global comparison of sites that elevated export productivity appears to be most common in tropical waters, which suggests that changing plankton ecology evidenced by the micrite layer altered the biological pump in a way that encouraged a temporary increase in export production in the tropics.

Sequential changes in ocean circulation and biological export productivity during the last glacial–interglacial cycle: a model–data study

We conduct a model–data analysis of the marine carbon cycle to understand and quantify the drivers of atmospheric CO2 concentration during the last glacial–interglacial cycle. We use a carbon cycle box model, “SCP-M”, combined with multiple proxy data for the atmosphere and ocean, to test for variations in ocean circulation and Southern Ocean biological export productivity across marine isotope stages spanning 130 000 years ago to the present. The model is constrained by proxy data associated with a range of environmental conditions including sea surface temperature, salinity, ocean volume, sea-ice cover and shallow-water carbonate production. Model parameters for global ocean circulation, Atlantic meridional overturning circulation and Southern Ocean biological export productivity are optimized in each marine isotope stage against proxy data for atmospheric CO2, δ13C and Δ14C and deep-ocean δ13C, Δ14C and CO32-. Our model–data results suggest that global overturning circulation weakened during Marine Isotope Stage 5d, coincident with a ∼ 25 ppm fall in atmospheric CO2 from the last interglacial period. There was a transient slowdown in Atlantic meridional overturning circulation during Marine Isotope Stage 5b, followed by a more pronounced slowdown and enhanced Southern Ocean biological export productivity during Marine Isotope Stage 4 (∼ −30 ppm). In this model, the Last Glacial Maximum was characterized by relatively weak global ocean and Atlantic meridional overturning circulation and increased Southern Ocean biological export productivity (∼ −20 ppm during MIS 3 and MIS 2). Ocean circulation and Southern Ocean biological export productivity returned to modern values by the Holocene period. The terrestrial biosphere decreased by 385 Pg C in the lead-up to the Last Glacial Maximum, followed by a period of intense regrowth during the last glacial termination and the Holocene (∼ 600 Pg C). Slowing ocean circulation, a colder ocean and to a lesser extent shallow carbonate dissolution contributed ∼ −70 ppm to atmospheric CO2 in the ∼ 100 000-year lead-up to the Last Glacial Maximum, with a further ∼ −15 ppm contributed during the glacial maximum. Our model results also suggest that an increase in Southern Ocean biological export productivity was one of the ingredients required to achieve the Last Glacial Maximum atmospheric CO2 level. We find that the incorporation of glacial–interglacial proxy data into a simple quantitative ocean transport model provides useful insights into the timing of past changes in ocean processes, enhancing our understanding of the carbon cycle during the last glacial–interglacial period.

Paleogene Polar Plankton and export productivity changes between the Eocene and Oligocene

<p>Aiming to support the prediction of future climate developments, this project investigates the role on geological timescale of the ocean plankton in reducing atmospheric carbon concentration by exporting carbon to the deep-sea. While it is well-known that the transition from the Eocene to the Oligocene brought significant climate changes and, in connection, also a change of the oceans’ carbon export production, the important role of phytoplankton and the links to changing ocean circulation are still poorly understood, as is, similarly, the impact on those changes on the diversity of the plankton contributing to the carbon pump. Investigating the nature of this interaction will provide significant insight into the functions of the oceans as climate regulators.</p><p>To address those question, we are generating diversity and absolute abundance data for diatoms and radiolarians, biogeographic data for radiolarians, as well as oxygen and carbon isotope data on planktic and benthic foraminifera, and on the fine fraction (<45µm, i. e. coccoliths), as well as other proxies to estimate surface and deep ocean temperatures and export productivity. These will be generated as paired data from individual samples in various deep-sea drilling sites in and around the Southern Ocean (as it is the focal point of the climatic/oceanographic changes at that period). These data will then be compiled and confronted to an ocean circulation model.</p><p>Here we will present our results so far (oxygen and carbon isotope on the bulk fine fraction, as well as radiolarian and diatom diversity estimates), based on two main localities from the antarctic (ODP Site 689B from the Weddell Sea) and the subantarctic (ODP Site 1090B on the southern flank of the Agulhas ridge) South Atlantic. A comparison with a newly generated, database-driven diversity analysis of the same groups in the same region, using the Neptune (NSB) database, will also be shown. While the exhaustive taxonomical compilation made on these two sites for the diatoms records three times more species than what was recorded in the literature for the Southern Ocean biome, it still shows an evolutionary turnover at the Eocene-Oligocene, just as the classic, NSB-driven analysis does. The fine fraction oxygen isotope at both sites 689B and 1090B show a pattern similar to that recorded in planktonic foraminifera in neighbour sites, indicating a significant drop in SST close to the Eocene-Oligocene boundary, while the fine fraction carbon isotope signal in the antarctic site shows a subsequent decrease indicating changes in exported productivity 2Myr after the global cooling.</p>