PALEOCEANOGRAPHIC IMPLICATIONS OF STABLE ISOTOPIC AND SEDIMENTOLOGIC CHANGE ACROSS THE MIOCENE/PLIOCENE BOUNDARY ON THE EAST TASMAN PLATEAU (ODP SITE 1172B)

A relatively complete Late Neogene section (Site 1172B; 44º 57.57’S, 149º 55.70’E) was recovered by ODP Leg 189 atop the East Tasman Plateau (water depth ~2630 m), providing an exceptional opportunity to study foraminiferal assemblage and sedimentological changes related to the late Miocene carbon isotope shift (~6.2 Ma). Site 1172B is located just north of the present-day subtropical convergence in subtropical-to-temperate waters. Stable isotope records compiled from size-specific specimens of planktonic (Globigerina bulloides, surface water; Globoconella clade, thermocline) and benthic (Planulina wuellerstorfi and Cibicidoides sp.) foraminiferal species show a general d¹³C decrease of ~0.5 per mil. Of particular interest is the collapse of the carbon isotope gradient between the planktonic and benthic species near the late Miocene/early Pliocene boundary indicating a well-mixed water column. In contrast, oxygen isotopic records display little to no significant change. Striking sedimentological changes coincide with the carbon isotope shift. Weight-percent sand (>63 µm) values decrease reflecting intensified carbonate dissolution through the interval of the carbon isotope shift. Parallel changes are also seen in planktonic foraminiferal assemblages, most notably a decline in the relative abundances of G. bulloides and Orbulina universa, which we attribute to a preservational bias due to species-specific dissolution. A number of mechanisms have been proposed for the cause of the marine carbon isotope shift (e.g., changes in ocean circulation, eustatic sea level fall). An alternative, but complementary, hypothesis is that the d¹³C decrease in the marine record may be a concomitant response to a d¹³C increase in the isotopic composition of the terrestrial reservoir of the carbon cycle (i.e. expansion of C4 grasses).