Can shallow marine carbonate faithfully preserve the true signal of carbon cycle perturbation?

The carbon cycle perturbations in geological history are preserved in the form of changes in stable carbon isotope ratios (δ¹³C values) in different carbon-bearing sedimentary archives. The carbon cycle perturbation that occurred across the Paleocene-Eocene boundary (~56 Ma) is known as the Paleocene Eocene thermal Maximum (PETM). After more than three decades of research, the exact magnitude of the negative carbon isotope excursion (CIE) is still fuzzy. The shallow marine sedimentary archive, deposited far above the lysocline, is considered to be the best archive to quantify the carbon cycle perturbation because the deep marine (carbonate) was likely to be affected by carbonate dissolution and terrestrial sedimentary records influenced by different climatic parameters. However, different biotic and abiotic processes could influence the magnitude of the CIE during the perturbed carbon cycle-climate state in a shallow marine environment. For this reason, the present study investigated the early Paleogene marine carbonate rocks deposited in the eastern Tethyan Sea (Ladakh, NW India) to check the possible presence of the PETM CIE and test whether shallow marine carbonate is a good archive for measuring the CIE magnitude. The presence of age-diagnostic larger benthic foraminifera and detailed micro-facies analysis indicates the investigated shallow marine carbonate rocks were deposited during the ~56 to 54 Ma (Shallow Benthic Zone - 4 to 7) and are likely to hold the PETM CIE. The secular variation in the δ¹³C values of unaltered bulk carbonate, screened through the cathodoluminescence microscopic study, reveals a PETM CIE magnitude of -3.6 ‰.  The observed CIE magnitude is similar to the globally accepted CIE magnitude (-4 ± 0.4 ‰) for PETM and suggests that shallow marine carbonate can be used to assess the magnitude of PETM and other carbon cycle perturbations.