Abstract:
A relatively complete Neogene section ( approximately 4.0-8.2 Ma) was recovered from the southern Tasman Sea (2,622 m), providing an excellent opportunity to investigate the responses of marine microplankton and carbonate chemistry to a permanent shift in the global dynamics of carbon cycling; an event referred to as the late Miocene carbon shift (LMCS). Parallel stable isotope records were generated through the study section using size-specific, depth-stratified planktonic (Globigerina bulloides, mixed-layer; Globoconella spp., thermocline) and benthic (Planulina wuellerstofi, Cibicidoides mundulus) foraminifera. All three delta (super 13) C curves record the LMCS, though the magnitude of this isotopic shift appears to decrease with increasing water depth. The onset of the LMCS predates the Tortonian/Messinian boundary, and coincides with magnetochron C4n.2n ( approximately 7.7 Ma). No significant increase is seen in the benthic delta (super 18) O record to indicate a substantial expansion of Antarctic ice sheets during the LMCS. The early stages of the LMCS are marked by a shoaling of the lysocline. This episode of intensified dissolution gave rise to a preservational bias among planktonic foraminiferal assemblages, most notably a decline in the relative abundance of G. bulloides that we attribute to species-selective dissolution. A curious aftereffect of the LMCS is that mixed-layer (G. bulloides) delta (super 13) C values converge upon benthic values, while thermocline (Globoconella spp.) values remain higher than benthic delta (super 13) C values. This delta (super 13) C inversion is due to a pronounced "vital effect" in the mixed-layer-dwelling G. bulloides that we attribute to either (1) increased uptake of metabolic (super 12) CO (sub 2) during calcification, or (2) increased [CO (sub 3) (super =) ] in seawater. The latter of these two explanations implies that oceanic pH increased following the LMCS, which may explain why post-LMCS carbonates take on a "modern" appearance with respect to their mean delta (super 13) C of Sigma CO (sub 2) .