Coupled productivity and carbon isotope records in the southwest Pacific Ocean during the late Miocene–early Pliocene biogenic bloom

Abstract

Biogenic components of sediment accumulated at high rates beneath frontal zones of the Indian and Pacific oceans during the late Miocene and early Pliocene. The δ¹³C of bulk and foraminiferal carbonate also decreased during this time interval. Although the two observations may be causally linked, and signify a major perturbation in global biogeochemical cycling, no site beneath a frontal zone has independent records of export production and δ¹³C on multiple carbonate phases across the critical interval of interest. Deep Sea Drilling Project (DSDP) site 590 lies beneath the Tasman Front (TF), an eddy-generating jetstream in the southwest Pacific Ocean. To complement previous δ¹³C records of planktic and benthic foraminifera at this location, late Neogene records of CaCO₃ mass accumulation rate (MAR), Ca/Ti, Ba/Ti, Al/Ti, and of bulk carbonate and foraminiferal δ¹³C were constructed at site 590. The δ¹³C records include bulk sediment, bulk sediment fractions (<63 µm and 5–25 µm), and the planktic foraminifera Globigerina bulloides, Globigerinoides sacculifer (with and without sac), and Orbulina universa. Using current time scales, CaCO₃ MARs, Ca/Ti, Al/Ti and Ba/Ti ratios are two to three times higher in upper Miocene and lower Pliocene sediment relative to overlying and underlying units. A significant decrease also occurs in all δ¹³C records. All evidence indicates that enhanced export production – the ‘biogenic bloom’ – extended to the southwest Pacific Ocean between ca. 9 and 3.8 Ma, and this phenomenon is coupled with changes in δ¹³C – the ‘Chron C3AR carbon shift’. However, CaCO₃ MARs peak ca. 5 Ma whereas elemental ratios are highest ca. 6.5 Ma; foraminiferal δ¹³C starts to decrease ca. 8 Ma whereas bulk carbonate δ¹³C begins to drop ca. 5.6 Ma. Temporal discrepancies between the records can be explained by changes in the upwelling regime at the TF, perhaps signifying a link between changes in ocean–atmosphere circulation change and widespread primary productivity.

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