Comparison between organic and inorganic stable carbon isotope ratios for understanding Pleistocene global and local climates, results from IODP Exp. 317 offshore South Island of New Zealand

Abstract:

IODP Expedition 317 drilled three sites on the continental shelf and one site on the slope of the offshore Canterbury, on the eastern margin of the South Island of New Zealand. Samples using this study were collected from Site U1352 located on the upper continental slope (water depth 344 m). Core recovery of this Site was nearly 100％ above 550m depth from the sea floor.

Biostratigraphic data suggest that the absence of the section from 2.7 to 1.8 Ma occurred between core depths 503 m and 523 m. Therefore, samples were taken from the upper 503 m of the Site U1352 at intervals of ~1.5 m, which represent ~7000 years duration since 1.8 Ma based on the average sedimentation rate of this portion. Benthic foraminifera Nonionella flemingiwere picked from samples and we measured oxygen and stable carbon isotope ratios of them. We also measured the stable carbon isotope ratios of organic matter and total organic carbon contents (TOC).

An age model was generated based on correlations between oxygen isotope ratios of this study and LR04 stack. We compared among the fluctuation curves of oxygen isotope ratios, inorganic and organic stable carbon isotope ratios, and TOC, using the age model. Fluctuations of stable carbon isotope and TOC derived from marine organic matters generally synchronize with oxygen isotope and stable carbon isotope records derived from benthic foraminifera. However, we found time lags among the fluctuation curves in some sections.

Fluctuation curves stem from organic matter might be controlled by the local productivity of the ocean surface, while isotope ratios of benthic foraminifera test depend on global climate change, such as glacier and inter-glacier cycles. Combined analysis of stable carbon isotope ratios of inorganic material and organic matter are considered to be a powerful tool for understanding the paleoenvironments.