High-resolution organic carbon isotopic chemostratigraphy and paleoclimatic conditions of Lower Aptian in the Tibetan Himalaya, southern Tibet 

Abstract

<p>The variations in carbon isotope (δ<sup>13</sup>C) values of the Aptian marine and terrestrial strata are globally comparable and regarded as an important tool for the stratigraphic correlation of the Aptian successions. The most remarkable feature of the δ<sup>13</sup>C curve of the Aptian is an abruptly negative excursion, which characterizes the onset of the early Aptian Oceanic Anoxic Event (OAE 1a, ~121 Ma). However, the identification of OAE 1a equivalent level in the Tibetan Himalaya remains debatable. Based on previously well-established foraminiferal biostratigraphy, we provide a lower Aptian high-resolution δ<sup>13</sup>C curve of bulk organic carbon from an expanded shale-dominated section, Chaqiela Section in the Tibetan Himalaya. The TOC/TN ratios of the studied section vary from 3 to 10, indicating that the organic matter is mainly sourced from marine plankton. Thus, we suggest that our results present the secular changes of δ<sup>13</sup>C in the dissolve carbon pool of the shallow sea on the south margin of eastern Tethys. The lower Aptian δ<sup>13</sup>C curve of the Chaqiela section is divided into eight segments, which can be well correlated to the representative sections in the western Tethys area.</p> <p>In the lower part of the section, an interval of sharp negative carbon isotope excursion (CIE) of ~1 ‰ is observed, which is followed by recovery of the δ<sup>13</sup>C values by a magnitude of ~2 ‰. This short-term carbon perturbation is superimposed on the long-term decrease in δ<sup>13</sup>C values of the early Aptian. The δ<sup>13</sup>C profile at this interval perfectly records the diagnostic characteristics of the OAE 1a: negative (C3), positive (C4), steady (C5), and positive (C6) segments. Lacking covariations of δ<sup>13</sup>C values with TOC/TN and TOC, and δ<sup>13</sup>C and Al in this interval indicate that the δ<sup>13</sup>C variations are not attributed to source changes of the organic matter. No obvious enrichments of the redox-sensitive major and trace elements indicate an oxic to sbuoxic condition during the sedimentation, it is in good agreement with the low C<sub>org</sub>:P<sub>tot</sub> ratios, but the slightly increase of C<sub>org</sub>:P<sub>tot</sub> ratios during the OAE 1a interval may imply a relatively oxygen-reducing trend and sustains till the end of OAE 1a. The paleoclimatic proxies imply a relatively warm and humid paleo-environment in mid-high latitudes (~50°S) of Tibetan Himalaya during this time interval, it is also testified by the moderate chemical weathering condition revealed by the A-CN-K ternary diagram.</p>