Late Cenozoic climate reconstruction from the Northern Indian Ocean using foraminiferal clumped isotope thermometry and stable isotope technique

Abstract

The Late Cenozoic era (23.03 million years ago to the present) offers a critical time window to examine the role of greenhouse gases, notably CO2 (ranging between 910 ppm and 180 ppm), in the operation of Earth's system processes. This study addresses important questions on convective rainfall and wildfire activity corresponding to three major climatic amelioration periods during the Late Cenozoic, based on sedimentary records from the Northern Indian Ocean.

Foraminiferal-clumped isotope (∆47) thermometry is employed to deduce Sea Surface Temperature (SST), and a new proxy of the stable nitrogen isotope ratio in clay-fixed ammonium (δ15Nf.NH₄⁺) is developed to assess the intensity of wildfire activity. An analytical milestone of inter-lab comparison of ∆47 and stable oxygen (δ18O) isotope using conventional gas source Isotope Ratio Mass Spectrometry (IRMS) and state-of-the-art Tunable infrared Laser Differential Absorption Spectrometer (TILDAS) techniques is achieved in this study for analysis of small sample size.

The above-mentioned techniques are employed to address the sensitivity of atmospheric CO2 levels on the rainfall over the Southern Bay of Bengal (SBoB) during the Miocene (between 9 – 18 Ma) time window, the response of South Asian Summer Monsoon (SASM) rainfall to the SST shift over Central-West BoB (CWBoB) during Glacial and Interglacial time window of past 31 kiloyears and, the ability of wildfire activity to withdraw atmospheric CO2 during the time window of the Plio-Pleistocene Transition (1.5 – 3 Ma).

The major findings indicate a statistically significant positive relationship between atmospheric CO2 levels and rainfall over the SBoB, with a sensitivity of a 3±1 psu decrease in Sea Surface Salinity (ΔSSS) per 100 ppm increase in CO2 during the Mid-Miocene time interval (13-17 Ma). This feedback was disrupted post-13 Ma due to the upliftment of the Eastern Tibetan Plateau. The second time window covering the past 31 kiloyears revealed a statistically significant relationship denoting a drop in ΔSSS by 0.9±0.1 psu in the NBoB for a 1°C rise in SST over CWBoB. The third time window of the Plio-Pleistocene transition (1.5 to 3 Ma) is marked by a drop of ⁓140 ppm in atmospheric CO2 level, leading to long-term global cooling. This decline is attributed to increased production, transport, and burial of organic carbon (OC) from terrestrial vegetation. The present study shows wildfire as a trigger for the uptake of atmospheric CO2 through the process of ecological shifts, increased soil erosion, and OC burial. Our results indicate a 1.5-fold increase in global OC burial rates, from 2.29±0.48 Mt C a-1 in the early Pliocene to 3.52±0.80 Mt C a-1 during the transition due to enhanced wildfire activity.