Evidence for Decreased Biological Carbon Pump Efficiency and Persistence of Photosymbiont Associations from Planktic Foraminifera across the Paleocene-Eocene Thermal Maximum

Abstract

The Paleocene-Eocene Thermal Maximum (PETM; ~56 million years ago) serves as an important analogue to the effects of rapid climate warming and carbon cycle perturbations. The key component to the global carbon cycle is the marine biological carbon pump (i.e., export production). The efficiency of which is dependent on the balance between the rate of organic carbon production and remineralization. Sustained surface ocean warming is expected to disrupt export production by reduced vertical mixing (stratification) and increasing rates of organic matter remineralization. Consequently, the flux of organic carbon through the water column is expected to decline, reducing the ocean’s capacity to efficiently sequester carbon and altering the structure of upper ocean ecosystems (e.g., depth stratification). Assessing changes in the efficiency of the biological carbon pump, and ocean and ecosystem structure over warming periods such as the PETM is of great importance given current global warming concerns. Here, I present novel high-resolution size-specific stable carbon and oxygen isotope records of planktic foraminifera from the South Atlantic (ODPS Site 1263, Walvis Ridge), spanning the PETM. Data from photosymbiont-bearing surface-dwelling (Acarinina and Morozovella) and asymbiotic thermocline-dwelling (Subbotina) planktic foraminifera is generated for the reconstruction of changes in the efficiency of the marine biological carbon pump across the PETM by evaluating surface-to thermocline and size-dependent carbon and oxygen isotopic gradients. The data reveals a decrease in the surface-to-thermocline δ13C- and δ18O-gradients during the CIE, suggesting a reduced efficiency of the biological carbon pump due to the shallower and more efficient remineralization of organic matter in the surface ocean. These findings support the hypothesis that PETM warming invoked a temperature-dependent increase in metabolic activity and remineralization rates in the mixed layer, which along with the effects of increased stratification resulted in decreased rates of export production in the open ocean. These findings are further supported by comparison of the δ13C- and δ18O-size covariance prior to and during the PETM, which reveals a possible inhabitance of shallower depths by Subbotina and narrow mixed-layer depth habitat for Acarinina and Morozovella, consistent with a shallower and more intense remineralization and reduced rated of food supply to the twilight zone. This thesis offers new insights into the importance and resilience of photosymbiont associations in symbiont-bearing planktic foraminifera to PETM warming and environmental perturbations, contrasting the opposing theory of symbiont bleaching or loss during the PETM at ODP 1263. Instead, it is proposed that twilight zone-dwelling taxa are subjected to high stress due to deoxygenation, reduced nutrient supply, and changing upper ocean ecosystem structure during extreme warming episodes such as the PETM, to which future adaptability is hard to predict. The rate of modern climate change exceeds that of the PETM by an order of magnitude, potentially leading to drastic changes in carbon pump efficiency and biological activity far beyond the scope of PETM perturbations.