LOCAL BUT PRIMARY ORIGIN OF THE EARLY CAMBRIAN CARBON ISOTOPE RECORD FROM MOROCCO

Carbon isotope excursions in Earth's history have been used to reconstruct changes in global carbon cycling and to correlate sedimentary strata intrabasinally and globally. However, the underlying assumption that these excursions represent global signals in the open ocean has not been adequately tested. Here, with multiple geochemical proxies (i.e., [Sr], δ^44/40Ca and δ²⁶Mg), we revisit the most expanded δ¹³C record of the early Cambrian from Morocco to investigate if this shallow-water carbonate record is representative of the open ocean, and what effect early marine diagenesis has had on the δ¹³C excursions used for stratigraphic correlation? Our [Sr] data show a linear correlation with δ¹³C, which is at odds with the large difference in their residence times if recording open-ocean signals — therefore either [Sr], δ¹³C, or both have been diagenetically altered. Two numerical diagenesis models applied to [Sr] and δ^44/40Ca indicate that early marine diagenesis did not substantially alter the δ¹³C signals, but did change [Sr]. Coherent δ²⁶Mg variations imply sediment-buffered dolomitization, consistent with the conclusion about the primary origin of the δ¹³C record. Therefore, while we do not have strong evidence that the δ¹³C signal reflects global seawater, we conclude that Morocco δ¹³C records at least a primary local signal that may be correlatable to other shallow water carbonate platforms globally.