Local Controls on Carbon Cycling in the Ordovician Midcontinent Region of North America, with Implications for Carbon Isotope Secular Curves

Abstract

The carbon isotope record from ancient epicontinental seas may contain much more of a local-scale carbon cycling signal than is generally appreciated. A unique opportunity exists to examine this issue in the case of the Late Ordovician Mohawkian Sea of eastern Laurentia, where the Millbrig K-bentonite stratigraphic framework has been used to delineate a time slice at 454 Ma, extending over ,1,500,000 km 2 of the eastern United States. Across the time slice, carbonate and organic carbon d 13 C vary by 4.5% and 7.5% respectively, a spatial variation that is as large as temporal (secular) changes in epeiricsea d 13 C that have been reported in the past. These new data are considered in the context of geographic variations in lithological, biological, and other geochemical sediment characteristics. Collectively, these sediment properties distinguish regions of the Mohawkian Sea which likely differed in terms of the nature and relative importance of carbon cycling processes. Water-column depth and structure, and barriers to free exchange of water across the Mohawkian Sea, may have been overarching factors in the development of these regions, raising the possibility that changes in circulation patterns, such as those caused by sea-level change, played a role in driving secular carbon isotope excursions by changing the rate of exchange of dissolved inorganic carbon between water masses. If the observed effects of local carbon cycling on the distribution of Mohawkian Sea d 13 C were commonplace in ancient epicontinental marine environments, it would imply that local-scale carbon cycling may have left a nontrivial imprint on epeiric-sea records of secular variations in d 13 C, in addition to the imprint left by changes in the global carbon cycle. This may have contributed to the broad scatter in d 13 C values observed in the Paleozoic portion of the global carbon isotope secular curve.

δ44/40Ca variability in shallow water carbonates and the impact of submarine groundwater discharge on Ca-cycling in marine environments

“…For example, some of the scatter observed in the putative record of calcium isotope variation in Phanerozoic oceans (Farkas et al, 2007), much of which is reconstructed from analyses of brachiopod shells collected from the deposits of epeiric seas, may be due to local Ca-cycling effects. A similar claim has been made for carbon isotope records reconstructed from epeiric sea carbonate successions (Patterson and Walter, 1994;Holmden et al, 1998;Immenhauser et al, 2003;Panchuk et al, 2005Panchuk et al, , 2006Melchin and Holmden, 2006;Fanton and Holmden, 2007;Immenhauser et al, 2008;Swart, 2008;LaPorte et al, 2009).…”

Chemocline instability and isotope variations of the Ediacaran Doushantuo basin in South China

“…In the modern carbonate platform in Florida, up to ≥4‰ variations in δ 13 C have been observed between marginal reef complexes and protected lagoons [32,33] . Similarly, an up to 4.5‰ variation in δ 13 C has been reported from time-equivalent depositional environments of the Late Ordovician Mohawkian Sea of eastern Laurentia [34] . However, such variations are commonly at shorter time scales than the Ediacaran While diagenetic influences on the isotope record of the Doushantuo Formation in South China and other Ediacaran successions in general should not be underestimated and require further investigation in a more detailed, bed-to-bed scales, available isotope data from Ediacaran successions do imply, in first order, the control from unique Ediacaran ocean chemistry.…”

The Species-Area Relationship in the Late Ordovician: A Test Using Neutral Theory

“…Holmden et al [2] used differences in ε Nd and δ 13 C in the Late Ordovician to identify strata that record deposition within the Appalachian Basin, Southern, and Midcontinent regions. In addition, differences in δ 13 C isotope values among these regions has been linked to different rates of carbon cycling [4], further supporting the evidence for limited mixing between water masses and linking these geochemical distinctions to biological activity. That these regions had distinct isotopic compositions suggests the presence of barriers to biological migration, and thus, that these areas would have been distinct biogeographic provinces.…”

“…Bathymetric differences across the Laurentian sea coupled with its vastness created oceanographically distinct geochemical regions [2][3][4]. Since these regions correspond to known faunal provinces, e.g., [13][14][15][16], they can be treated as separate metacommunities, or biogeographic provinces: the Appalachian Basin, Southern, Midcontinent, and Western provinces (Figure 3).…”

“…Since there was likely little mixing between the water masses of these regions [4], each can be treated as a single metacommunity or a province that experiences little or no biotic interchange with adjacent metacommunities [1]. By dividing the Laurentian epicontinental sea in this manner, it is possible to test for the predicted linear relationship between θ and metacommunity area.…”