Sea level rise, shallow carbonate deposition, ocean alkalinity and climate change during Cenozoic

During Quaternary,the sea level around Borneo has oscillated, periodically connecting and disconnecting Borneo from continental Asia (Earl of Cranbrook, 2009). The climate of this region has subsequently undergone significant changes. Abrams et al. (2018) proposed that shallow carbonate deposition in flooded shelves of the Sundaland could have resulted in an increase of pCO₂ by 39 ppm during Holocene (since 5000 BP). A thorough understanding of the processes that link the "shallow carbonate deposition" to atmospheric CO₂ needs further examination. Carter et al. (2014) have introduced a composite indicator, Alk^*, primarily determined by calcium carbonate precipitation or dissolution. They have shown that the effect of temperature on CO₂ solubility, freshwater influence and disequilibrium in the air-sea exchange of CO₂ (in their order of decreasing influence) significantly influences the calcite saturation state in the seawater. The calcium carbonate cycling has an almost insignificant role. Thus, changes in the calcite saturation state are mutually adjusted by the cycling of calcium carbonate by attaining an equilibrium for the given temperature and salinity. The calcite saturation state exerts control over CO₂ uptake by the ocean. Saturation leads to calcite precipitation and, undersaturation leads to dissolution. Precipitation of calcite transforms atmospheric CO2 into sediment repository that maybe finally subducted and undergo a long term burial that may outgas during arc magmatism. Calcium carbonate cycling does not have control over calcite saturation state and therefore ocean alkalinity. Then, the conclusion that silicate weathering is not responsible for long term Cenozoic cooling (Moore et al., 2013) may be an artefact of the inability of the calcium carbonate to influence the alkalinity of the ocean significantly.

References:

Abrams, J.F., Hohn, S., Rixen, T., Merico, A., 2018. Sundaland peat carbon dynamics and its contribution to the Holocene atmospheric CO₂ concentration. Global Biogeochemical Cycles 32, 704–719. https://doi.org/10.1002/2017GB005763

Carter, B.R., Toggweiler, J.R., Key, R.M., Sarmiento, J.L., 2014. Processes determining the marine alkalinity and calcium carbonate saturation state distributions. Biogeosciences 11, 7349–7362. https://doi.org/10.5194/bg-11-7349-2014

Earl of Cranbrook, 2010. Late quaternary turnover of mammals in Borneo: the zooarchaeological record. Biodivers Conserv 19, 373–391. https://doi.org/10.1007/s10531-009-9686-3

Moore, J., Jacobson, A.D., Holmden, C., Craw, D., 2013. Tracking the relationship between mountain uplift, silicate weathering, and long-term CO₂ consumption with Ca isotopes: Southern Alps, New Zealand. Chemical Geology 341, 110–127. https://doi.org/10.1016/j.chemgeo.2013.01.005