Middle Eocene Climatic Optimum's Ocean Productivity Spiked Because of Increased Eolian Iron Fertilization

Environmental magnetic implications of magnetofossil occurrence during the Middle Eocene Climatic Optimum (MECO) in pelagic sediments from the equatorial Indian Ocean

Authors:

Savian et al

Abstract:

Magnetic properties of pelagic marine sediments that record the Middle Eocene Climatic Optimum (MECO) at ~ 40 Ma provide information about major environmental changes. The main variations observed during this transient warming event reflect a bacterial magnetofossil signal, but the cause of the linkage between bacterial production and climate remains unclear. We present an environmental magnetic study of middle Eocene deep-sea sediments from the northern edge of Madingley Rise (Ocean Drilling Program Hole 711A, equatorial Indian Ocean) to investigate the origin of the increased magnetic mineral concentration concomitant with subchron C18n.2n, which corresponds to the MECO interval in ODP Hole 711A. This magnetic mineral peak also coincides with a change in lithofacies from calcareous nannofossils to radiolarian ooze, and a slight increase in clay concentration. Magnetite is the main magnetic mineral in the MECO sediments, which occurs as magnetically non-interacting single domain biogenic particles. The increased magnetic mineral concentration across the MECO event is likely to have been caused by increased eolian iron fertilization. This is interpreted to have given rise to increased surface ocean productivity, where increased delivery of iron and nutrients to the seafloor enhanced magnetotactic bacterial populations during the MECO event.

Marine Biota Response to the Middle Eocene Climatic Optimum

Benthic foraminiferal response to the Middle Eocene Climatic Optimum (MECO) in the South-Eastern Atlantic (ODP Site 1263)

Authors:

Galazzo et al

Abstract:

The response of marine biota to the Middle Eocene Climatic Optimum (MECO) is still poorly constrained. Specifically, changes in deep-sea benthic foraminiferal faunas have been documented at few locations only. We carried out a quantitative study of benthic foraminiferal assemblages at lower-bathyal ODP Site 1263 (Walvis Ridge, SE Atlantic), providing documentation of the response of benthic foraminiferal assemblage to the MECO in an open ocean setting. There was no major temporary or persistent assemblage turnover in benthic foraminiferal faunas in the SE Atlantic Ocean. The benthic foraminiferal accumulation rates (BFAR) and assemblage composition indicate that the delivery of food to the seafloor increased during the early stages of surface and deep-sea warming. During this period, the absolute and relative abundance of phytodetritus exploiters increased pronouncedly, indicating that the increase in the flux of organic matter to the sea floor was largely seasonal or otherwise pulsed. During later stages and peak warming of MECO, in contrast, the flux of organic matter to the seafloor declined markedly, as shown by a decrease in both infaunal and epifaunal benthic foraminiferal accumulation rates. The low flux of organic matter during the more extreme parts of the MECO thus did not favor one of these broad groups, but affected all common taxa. Paleoceanographic reconstructions combined with data on benthic foraminifera suggest that the MECO warming was the main cause of the reduction in the flux of organic matter to the sea floor, by increasing the metabolic rates of pelagic consumers, which thus used more food, preventing it from reaching the sea floor, as well as increasing remineralization of organic matter in the water column.

Evidence of the Middle Eocene Climatic Optimum From the South-East Atlantic

The Middle Eocene Climatic Optimum (MECO): A multi-proxy record of paleoceanographic changes in the South-East Atlantic (ODP Site 1263, Walvis Ridge)

Authors:

Boscolo Galazzo et al

Abstract:

The Middle Eocene Climatic Optimum (MECO, ~40 Ma) was a transient period of global warming that interrupted the secular Cenozoic cooling trend. We investigated the paleoceanographic, paleoenvironmental and paleoecological repercussions of the MECO in the south-east Atlantic subtropical gyre (Ocean Drilling Program Site 1263). TEX86 and δ18O records support an ~4 °C increase in surface and deep-water temperatures during the MECO. There is no long-term negative carbon isotope excursion (CIE) associated with the early warming, consistent with other sites, and there is no short-term negative CIE (~50 kyr) during the peak of the MECO, in contrast to what has been observed at some sites. This lack of a CIE during the peak of the MECO at Site 1263 could be due to poor sediment recovery or geographic heterogeneity of the δ13C signal. Benthic and planktic foraminiferal mass accumulation rates declined markedly during MECO, indicating a reduction of planktic foraminiferal production and export productivity. Vertical δ13C gradients do not indicate major changes in water-column stratification, and there is no biomarker or micropaleontological evidence that hypoxia developed. We suggest that temperature-dependency of metabolic rates could explain the observed decrease in foraminiferal productivity during warming. The kinetics of biochemical reactions increase with temperature, more so for heterotrophs than for autotrophs. Steady warming during MECO may have enhanced heterotroph (i.e., foraminiferal) metabolic rates, so that they required more nutrients. These additional nutrients were not available because of the oligotrophic conditions in the region and the lesser response of primary producers to warming. The combination of warming and heterotroph starvation altered pelagic food webs, increased water-column recycling of organic carbon, and decreased the amount of organic carbon available to the benthos.