Changes and influencing factors in biogenic opal export productivity in the Bering Sea over the last 4.3 Ma: Evidence from the records at IODP Site U1340

Abstract

AbstractWe reconstructed changes in biogenic opal export productivity (BOEP) in the southern Bering Sea (BS) over the last ∼4.3 Ma, based on mass accumulation rate (MAR) of biogenic opal from Integrated Ocean Drilling Program (IODP) Site U1340. The results show that the BOEP in the BS was high and variable between ∼4.3 and ∼1.9 Ma, extremely low and relatively stable from ∼1.9 to ∼1.1 Ma, and then fluctuated frequently (generally high during interglacials and low during glacials) during the last ∼1.1 Ma. One interval of enhanced BOEP from ∼4.3 to ∼3.2 Ma is a response to the Late Miocene‐Early Pliocene “Biogenic Bloom Event.” Another interval from ∼2.8 to ∼1.9 Ma correlates with global opal burial shifting from high‐latitude oceans to upwelling‐influenced regions following the intensification of the Northern Hemisphere Glaciation (NHG). Whereas, the increase in BS opal export productivity during the last ∼1.1 Ma tends to be a “local” phenomenon. Overall, the BOEP shows a similar trend and good correspondence to the input of the Alaskan Stream (AS), which can be traced using the Na2O/K2O ratio. We thus conclude that the AS may be the direct, and primary factor on BOEP variability in the BS during the last ∼4.3 Ma. In addition, although the poor correlation between opal MAR and volcanic glass suggests that BOEP variability was not controlled by long‐term variations in the volcanism or ash abundance, increased ash abundance indicated by high contents of volcanic glasses was also a possible reason for enhanced BOEP during the period from ∼4.3 to ∼3.2 Ma and the last ∼0.5 Ma.