The middle Eocene climatic optimum (MECO): A multiproxy record of paleoceanographic changes in the southeast Atlantic (ODP Site 1263, Walvis Ridge)

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 southeast Atlantic subtropical gyre (Ocean Drilling Program Site 1263). TEX 86 and δ 18 O records support an~4°C increase in surface and deepwater 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 δ 13 C signal. Benthic and planktic foraminiferal mass accumulation rates markedly declined during MECO, indicating a reduction of planktic foraminiferal production and export productivity. Vertical δ 13 C 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.

Episodes of intensified biological productivity in the subtropical Atlantic Ocean during the termination of the Middle Eocene Climatic Optimum (MECO)

“…The abundance of planktic foraminifera has been shown to exhibit a strong positive correlation with primary productivity [ Thunell and Reynolds , ; Van Kreveld , ]. Thus, a number of studies have used this correlation to make quantitative assumptions about paleoproductivity based on planktic foraminiferal abundances, often in combination with BFARs and the accumulation rates of other marine organisms [e.g., Boscolo Galazzo et al ., ; Diester‐Haass and Zahn , ; Hebbeln et al ., ; Melki et al ., ; Rasmussen et al ., , ; Van Kreveld , ].…”

“…However, the post‐MECO interval is not represented in this section, hindering further correlation with this data set. New records from Walvis Ridge in the eastern South Atlantic (Site 1263) also show evidence for increased productivity associated with the MECO [ Boscolo Galazzo et al ., , ]. Although the composition of benthic foraminiferal assemblages was not significantly affected by the environmental changes during the MECO, benthic and planktic foraminiferal abundances both increase during the MECO at Site 1263 [ Boscolo Galazzo et al ., ], similar to our results at Site 1051.…”

“…Although the composition of benthic foraminiferal assemblages was not significantly affected by the environmental changes during the MECO, benthic and planktic foraminiferal abundances both increase during the MECO at Site 1263 [ Boscolo Galazzo et al ., ], similar to our results at Site 1051. However, peaks in foraminiferal accumulation rates at Site 1263 precede the peak‐warming interval (and HPI‐1) by several thousand years, which is attributed to foraminiferal starvation due to increases in the metabolic rates at this site in response to warming [ Boscolo Galazzo et al ., ].…”

“…At subtropical Site 1051 and in the Tethyan Alano section, the increase in trophic state seems to be restricted to the peak‐warming interval, while eutrophication begins much earlier at 40.4 Ma (300 kyr before peak warming) in the Southern Ocean and southern Atlantic. The response of the benthic foraminiferal communities ranges from very strong at Site 738 (in form of a faunal turnover) [ Moebius et al ., ], to apparent but less pronounced in the Tethys [ Boscolo Galazzo et al ., ], to muted at Site 1051 (this study) and Site 1263 (South Atlantic) [ Boscolo Galazzo et al ., , ]. Furthermore, it appears that different microfossil groups (benthic and planktonic foraminifera, diatoms, radiolarians) reacted differently to a similar environmental perturbation, which might be attributed to the specific local environmental conditions at each site.…”

“…Assuming an absence of significant continental ice sheets at that time [ Bijl et al ., ; Burgess et al ., ; Edgar et al ., ], this δ 18 O shift likely represents a temperature increase of 4–6°C in surface and deep waters [ Bohaty and Zachos , ; Bohaty et al ., ; Edgar et al ., ; Sexton et al ., ]. This is consistent with estimates from independent temperature proxies such as TEX 86 and U K 37 , suggesting high‐latitude sea surface warming of 3°C to 6°C during the MECO [ Bijl et al ., ; Boscolo Galazzo et al ., ]. Inferred ocean warming culminates in a δ 18 O minima (i.e., MECO peak warming) lasting ~50 kyr at ~40.0 Ma [ Bohaty et al ., ; Edgar et al ., ].…”