Silicic acid cycling in the Bering Sea during the Mid‐Pleistocene Transition

Abstract

The rate of deep-ocean carbon burial is considered important for modulating glacial-interglacial atmospheric CO2 concentrations and global climate during the Quaternary. It has been suggested that glacial iron fertilization and increased efficiency of the biological pump in the Southern Ocean since the Mid-Pleistocene Transition (MPT) was key in lowering atmospheric pCO2 and facilitating rapid land ice accumulation. There is growing evidence that a similar mechanism may have existed in the subarctic Pacific Ocean, although this has not yet been assessed. Here, the silicon isotope composition of diatoms (δ30Sidiatom) from the Bering Sea upwelling region is used to assess the role of nutrient cycling on the subarctic Pacific biological pump during the MPT. Results show that during and after the “900 kyr event,” the high productivity green belt zone was characterized by low silicic acid utilization but high supply, coincident with the dominance of diatom resting spores. We posit that as nutrient upwelling was suppressed following pack ice growth and expansion of glacial North Pacific Intermediate Water (GNPIW), primary productivity became nitrate-limited and enhanced opal remineralization caused a relative increase in silicic acid supply. However, preferential preservation and higher cellular carbon content of diatom resting spores, as well as increased supply of iron from expanded sea ice, likely sustained the net efficiency of the Bering Sea biological pump through the MPT. Remnant iron and silicic acid may also have propagated into the lower subarctic Pacific Ocean through GNPIW, aiding a regionally efficient biological pump at 900 kyr and during post-MPT glacials.