Paper No. 6
Presentation Time: 9:15 AM
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 δ13
C 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 δ13
C values ranging from -0.2 to -17.1 and δ18
O from -0.3 to -10.0. Furthermore, individual sequences have their own distinct δ13
O clusters; and field observation indicates that the most 13
C-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 δ13
C 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.