Late Neogene biogenic sedimentation and carbon isotope shifts in the southwest Pacific Ocean

• Main Author: Grant, Katharine Mary
• Format: Thesis
• Language: unknown
• Published: 2005
• Subjects: Antarctic; Pacific; Indian; Antarc*
• Online Access: https://researchonline.jcu.edu.au/32251/1/32251_Grant_2005_thesis.pdf

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 foraminiferan 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. Furthermore, most of the δ¹³C data for the late Miocene-early Pliocene are not calibrated on the same timescale. In the Southwest Pacific Ocean, Deep Sea Drilling Project (DSDP) site 590 and Ocean Drilling Program (ODP) site 1125 lie beneath major frontal zones: the Tasman Front and Subtropical Convergence (STC), respectively; DSDP site 594 lies just outside the STC. Late Neogene records of CaCO₃ mass accumulation rate (MAR), bulk carbonate δ¹³C, and Ca/Ti, Si/Ti and Al/Ti ratios have been constructed at all three sites, and are complemented at sites 590 and 1125 by δ¹³C records of planktic and benthic foraminifera; a record of Ba/Ti has also been produced for site 590. At site 590, CaCO₃ MARs, Ca/Ti, Al/Ti and Ba/Ti ratios are 2 to 3 times higher in upper Miocene and lower Pliocene sediment relative to overlying and underlying units. A significant decrease (between 1 and 2 ‰) also occurs in all δ¹³C records. All evidence indicates that enhanced export production - the "biogenic bloom" - extended to the Tasman Front 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; foraminiferan δ¹³C starts to decrease ca. 8 Ma whereas bulk carbonate δ¹³ begins to drop ca. 5.6 Ma. Temporal discrepancies between the records can be explained by changes in the upwelling regime at the Tasman Front and by depth differences between coccolithophorid and foraminiferan carbonate precipitation. The sediment records from site 1125 show very similar trends to those from site 590. CaCo₃ MARs are eight-fold higher between 5.66 and 5.38 Ma, relative to average Neogene values, and this maximum coincides with elevated Ca/Ti ratios and faunal abundance patterns indicative of elevated upwelling and primary productivity. These observations, and their absence from the site 594 records, are interpreted as evidence of the biogenic bloom in the STC. The suite of δ¹³C records from site 1125 all show significantly lower values for the early Pliocene compared to the latest Miocene, and the depletion occurs mainly in two distinct shifts. The "Chron C3Ar carbon shift" (between –0.6 and –1.5 ‰ magnitude) is recorded by planktic and benthic foraminifera between 7.0 and 6.4 Ma, and an "early Gilbert carbon shift" is recorded by both the bulk sediment (–1.8 ‰; 5.1-3.6 Ma) and all species of foraminifera (~ –1.0 ‰; 5.3-4.9 Ma). Bulk sediment from site 594 also decreases markedly (–1.7 ‰) between 5.0 and 3.6 Ma. These isotopic excursions are explained by coupled variations in hydrography, productivity, and depth of calcite precipitation, as well as an increase in terrigenous fluxes to Chatham Rise ca. 5.4 Ma. The overall driving force of the biogenic bloom and late Neogene carbon isotope shifts in the southwest Pacific Ocean can be related to climate-driven changes in ocean circulation, probably arising from Antarctic glaciation.