Strontium and carbon isotope stratigraphy of the Late Jurassic shallow marine limestone in western Palaeo-Pacific, northwest Borneo

Strontium and carbon isotope stratigraphy was applied to a 202m-thick shallow marine carbonate section within the Late Jurassic Bau Limestone at the SSF quarry in northwest Borneo, Malaysia, which was deposited in the western Palaeo-Pacific. Strontium isotopic ratios of rudist specimens suggest that the SSF section was formed between the latest Oxfordian (155.95Ma) and the Late Kimmeridgian (152.70Ma), which is consistent with previous biostratigraphy. The δ¹³C_carb values of bulk carbonate range from -0.10 to +2.28% and generally show an increasing upward trend in the lower part of the section and a decreasing upward trend in the upper part of the section. A comparable pattern is preserved in the δ¹³C_org isotope record. Limestone samples of the SSF section mainly preserve the initial δ¹³C_carb values, except for the interval 84-92m, where an apparent negative anomaly likely developed as a result of meteoric diagenesis. Comparing with the Tethyan δ¹³C_carb profile, a negative anomaly in the lower SSF section can be correlated with the lowered δ¹³C values around the Oxfordian/Kimmeridgian boundary. In addition, δ¹³C_carb values of the Bau Limestone are generally ~1% lower than the Tethyan values, but comparable with the values reported from Scotland and Russia, located in Boreal realm during the Late Jurassic. This suggests that either the Tethyan record or the other records have been affected by the δ¹³C values of regionally variable dissolved inorganic carbon (DIC). The Late Jurassic δ¹³C_DIC values are thought to have been regionally variable as a result of their palaeoceanographic settings. This study shows that δ¹³C chemostratigraphy of the Palaeo-Pacific region contributes to an improved understanding of global carbon cycling and oceanography during this time period.