HOW RELIABLE IS THE CARBON ISOTOPIC COMPOSITION OF PALEOZOIC CHITINOPHOSPHATIC SHELL MATERIAL? A CASE STUDY USING CAMBRIAN (MARJUMAN - STEPTOEAN) BRACHIOPODS

Paleozoic carbon cycle reconstructions using carbon isotopes can be undertaken with a variety of geological materials. While a majority of studies use carbonate matrix cement or organic matter from bulk rock samples, other studies have focused on unaltered carbonate and phosphatic skeletal material as a means of ensuring that the carbon isotopic composition is indeed primary and/or because no carbonate cement is present. In the Cambrian (Marjuman – Steptoean) succession of the Upper Mississippi Valley (UMV), the dominant lithology is friable quartz and glauconitic sandstones and shales. Though dolomite cement is observed in some intervals, its occurrence is not consistent enough stratigraphically to reconstruct an interpretable carbon isotopic profile using carbonate matrix cement. For this reason, previous isotopic studies in the UMV have relied on chitinophosphatic brachiopod shell material to reconstruct carbon cycle changes like the SPICE (Steptoean Positive Isotope Carbon Excursion) Event.

In this case study we test the reliability of chitinophosphatic brachiopods from the UMV for preserving primary carbon isotopes by comparing δ¹³C isotopic trends and absolute values of chitinophosphatic shell material from nearby cores, comparing the δ¹³C values of chitinophosphatic shell material to that of dolomitic cement of bulk rock matrix surrounding the shells in both core and outcrop, and by comparing the δ¹³C composition of shells of different color from a partially weathered outcrop bedding surface. Our results indicate that 1) the SPICE can be reconstructed among different cores with regard to both isotopic trends and absolute values, 2) chitinophosphatic shell material records an ~1-2‰ lighter biogenic fractionation (vital effect) when compared to surrounding matrix carbonate, and, 3) substantial color alteration of chitinophosphatic shell material due to surface weathering resulted in an isotopic shift or <1‰. This case study indicates that chitinophosphatic brachiopods reliably preserve (with a vital effect) the primary carbon isotopic composition of Cambrian seawater.