HIGH SPATIAL HETEROGENEITY OF A CAP-CARBONATE’S STABLE ISOTOPE SIGNATURES: A STAGNANT LATE NEOPROTERZOIC OCEAN BASIN

Thin (~ 2-5 m) dolomicrite sequences capping a possible post-Marinoan glacial diamictite, the Hankalchough Formation, eastern Chinese Tianshan, have one of the most negative δ¹³C values ever reported for bulk carbonates. Intriguingly, four of the same cap-carbonate sequences from spatially different sections (L, ZBS, YKG, and MK) within an area of ~ 2000 km² exhibit a great heterogeneity in both carbon and oxygen isotope compositions, with δ¹³C values ranging from -0.2 to -17.1‰ and δ¹⁸O from -0.3 to -10.0‰. Furthermore, individual sequences have their own distinct δ¹³C-δ¹⁸O clusters; and field observation indicates that the most ¹³C-depleted sequence (ZBS) caps the thickest diamictite suite, while the least depleted one (YKG) caps the thinnest diamictite among the four. We propose that the observed spatial heterogeneity is strong evidence for a stagnant (or stratified) ocean basin. The extremely negative δ¹³C values could be achieved when nearly half of the HCO₃^- in an anoxic water body were derived from organic matter through microbial sulfate-reduction. Spatial variability in carbon and oxygen isotope compositions reflects the different water depths at which the cap dolomicrites formed.

This model requires highly-intensive sulfate reduction and limited vertical exchange of water in the ocean basin. These constraints predict a small difference in δ³⁴S values between disseminated pyrite in the carbonate and sulfate in anoxic bottom water. In addition, a dolomicrite sequence deposited at intermediate water depths should have a larger variability in its pyrite δ³⁴S values than a sequence deposited in deep anoxic water of a stratified ocean basin. We test this hypothesis by analyzing the δ³⁴S of disseminated pyrite in the four cap-carbonate sequences. The two extremely ¹³C-depleted sequences (L and ZBS) have more positive and less variable δ³⁴S values (14.7 ± 2.7‰, and 9.9 ± 4.0‰, respectively) than those from the less ¹³C-depleted sequence (MK) (-1.3 ± 9.6‰), a result consistent with the stagnant ocean basin model. The YKG sequence has the highest average δ¹³C value but possess pyrite δ³⁴S values (12.9 ± 5.0‰) similar to those from the two extremely ¹³C-depleted sequences (L and ZBS). Further test of the stagnant ocean model requires analyses of additional Hankalchough sequences in the region.