Early Cenozoic Decoupling of Climate and Carbonate Compensation Depth Trends

Our understanding of the long-term evolution of the Earth system is based on the assumption that terrestrial weathering rates should respond to, and hence help regulate, atmospheric CO2 and climate. Increased terrestrial weathering requires increased carbonate accumulation in marine sediments, which in turn is expected to result in a long-term deepening of the carbonate compensation depth (CCD). Here, we critically assess this long-term relationship between climate and carbon cycling. We generate a record of marine deep-sea carbonate abundance from selected late Paleocene through early Eocene time slices to reconstruct the position of the CCD. Although our data set allows for a modest CCD deepening, we find no statistically significant change in the CCD despite >3 degrees C global warming, highlighting the need for additional deep-sea constraints on carbonate accumulation. Using an Earth system model, we show that the impact of warming and increased weathering on the CCD can be obscured by the opposing influences of ocean circulation patterns and sedimentary respiration of organic matter. From our data synthesis and modeling, we suggest that observations of warming, declining delta C-13 and a relatively stable CCD can be broadly reproduced by mid-Paleogene increases in volcanic CO2 outgassing and weathering. However, remaining data-model discrepancies hint at missing processes in our model, most likely involving the preservation and burial of organic carbon. Our finding of a decoupling between the CCD and global marine carbonate burial rates means that considerable care is needed in attempting to use the CCD to directly gauge global carbonate burial rates and hence weathering rates. Plain Language Summary Weathering, the breakdown of rocks at the Earth's surface, is widely assumed to act as Earth's thermostat, regulating the concentration of atmospheric carbon dioxide (CO2) and global temperatures. This is because (a) weathering consumes CO2 from the atmosphere and (b) weathering rates are thought to be greater at higher temperatures. One widely used proxy for reconstructing global weathering is the preservation of calcium carbonate (chalk) minerals in the deep ocean. From the late Paleocene to early Eocene, the planet warmed by more 3 degrees C, the biggest long-term global warming trend for which we have detailed records. It is still uncertain whether global weathering responded to this temperature increase. We reconstruct the preservation of deep-sea calcium carbonate across this warming and find little apparent change. We also use computer modeling to show that the relationship between global weathering and preservation of deep-sea calcium carbonate is highly nuanced. In sum, we are able to attribute the late Paleocene to early Eocene warming to an increase in CO2 emissions from volcanoes. We argue the resulting elevated temperatures did, in fact, cause an increase in global weathering, but that this is not readily apparent from our deep-sea calcium carbonate proxy.