A PULSE OF MARINE ORGANIC CARBON TO THE ABYSS: A(NOTHER) POSSIBLE MECHANISM FOR HIGH d13C VALUES DURING THE LATE ORDOVICIAN GLACIAL EPOCH?

Several mechanisms have been proposed to explain high d¹³C values during the end Ordovician glaciation. Recent hypotheses have focused on continental weathering as a key mechanism for initiating glaciation and generating the Hirnantian d¹³C excursion. However, the roles of marine organic carbon oxidation and/or enhanced transfer to the deep ocean have not been fully introduced into the models, and may be a significant factor in explaining the extraordinarily high d¹³C values found in conjunction with the end Ordovician glacial event.

At Vinini Creek (Roberts Mountains, NV, USA), a succession of dark, high TOC shales is capped by a ~1m of rusty-weathering shales with low TOC values immediately below rising d¹³C values in overlying carbonate units. At Copenhagen Canyon (Monitor Range, NV), lithologies change from a succession of chert/carbonate interbeds to ~10m interval of heterogeneous grainstones overlain by massive carbonate units. A rapid rise in d¹³C values begins within the transition from grainstone to massive carbonate. Together, these successions suggest a brief period where stable oceanographic conditions were disrupted by more oxygenated, warmer? waters in conjunction with rapid lowering of sea level. A  lowering of d¹³C values during deposition of the rusty-weathering shales at Vinini Creek is compatible with an interpretation of enhanced organic oxidation.

It appears that baseline evolution of marine d¹³C values remained constant throughout the high d¹³C event. This suggests most factors controlling marine d¹³C values remained unchanged during the glacial event, and that a single transfer of low-d¹³C carbon to an isolated reservoir may be responsible for high  d¹³C values. We suggest that oxygenated waters  flooded continental shelves during invigoration of vertical ocean circulation, accompanied by a brief high flux of low-d¹³C carbon to the deep ocean. The resulting high d¹³C values remained high until a subsequent invigoration of circulation homogenized the ocean-atmosphere system and restored marine d¹³C to its value prior to the glacial event.