The stable carbon isotopic composition of organic material in platform derived sediments: implications for reconstructing the global carbon cycle

Abstract

In pelagic carbonate sediments, the degree to which the δ13C values of inorganic and organic fractions co‐vary has been used to interpret rates of production, burial and decomposition of organic carbon. This relationship is relatively consistent through time, permitting estimates of organic carbon production and preservation. However, as the majority of pelagic sediments older than 200 Myr have been subducted, carbonate sediments deposited in epeiric seas and platforms are often substituted for pelagic carbonates in analyses of ancient global carbon cycling. There are well‐known pitfalls to using shallow marine carbonate materials, including diagenesis, semi‐isolation of depositional environments and input of different types of sediments with varying inorganic δ13C (δ13Cinorganic) values, which can obscure any global signatures. One method used to assess whether global changes in δ13C are accurately represented by δ13Cinorganic records is to examine variations in the δ13C of co‐occurring organic material (δ13Corganic). If a δ13Corganic record co‐varies with a co‐occurring δ13Cinorganic record, it is argued that the signals must be related to variations in the global carbon cycle. This assumption has been investigated by analysing the isotopic composition of the organic carbon preserved in the uppermost 150 m of periplatform sediments recovered during ODP Leg 166 from the western margin of Great Bahama Bank. The δ13Corganic values measured in this study were compared to previously published δ13Cinorganic records measured on identical samples, thus allowing a study of the correlation between the two records through time. These analyses showed that the correlation coefficient between δ13Cinorganic and δ13Corganic increased from the proximal location (Site 1005, r2 = 0·1), to the distal site (Site 1006, r2 = 0·63). The importance of platform‐derived carbonate and organic material at the proximal location, Site 1005, is reflected in the absence of a co‐variation between inorganic and organic δ13C records, which exhibit no correlation on the platform itself. In contrast, the co‐variance in δ13C values at the basinal location, Site 1006, is explained by a two‐point mixing model, which demonstrates the importance of both pelagic and platform‐derived carbonate and organic carbon in generating the positive correlation between the organic and inorganic δ13C values; this results in a correlation between δ13Cinorganic and δ13Corganic records at Site 1006 that is unrelated to global carbon cycling. Such data question the applicability of using δ13Corganic values to support the ability of δ13Cinorganic values to record global carbon cycling in carbonates recovered from environments where multiple sources of carbonate and organic carbon contribute to the bulk δ13C signal.

Interpreting carbonate and organic carbon isotope covariance in the sedimentary record

“…The sediments and organic matter preserved closer to 100 mbmp exhibit low d 13 C carb and d 13 C org values, because they have been repeatedly affected by freshwater diagenetic reactions and postdepositional contributions of terrestrial organic matter, as previously described. In contrast, the d 13 C carb and d 13 C org values in the section of the core closer to 200 mbmp are comparatively more positive, and similar to those observed both on the modern platform top 45 as well as those preserved in the marine burial zone (Fig. 4), suggesting that this section of the core has experienced fewer episodes of alteration and lower contributions, if any, of terrestrial organic carbon.…”

“…3b). Both the d 13 C carb and d 13 C org values are significantly lower than those reported for modern shallow marine sediments from Great Bahama Bank, which average þ 4.5 and À 12%, respectively 44,45 . We suggest that these low d 13 C carb values arise from the oxidation of organic matter, which imparts a low d 13 C value to the dissolved inorganic carbon, along with cementation, mineralogical stabilization and recrystallization of the carbonate 33,39 .…”

Carbon isotope stratigraphy of the basal Zechstein (Lopingian) strata in Northern Poland and its global correlation

“…Typ i cally, if both the d 13 C of the in or ganic and or ganic por tions co-vary in a sed i men tary se quence, it is as sumed that the vari a tions in the d 13 C in or ganic val ues in di cate a real change in the d 13 C in or ganic of the en vi ron ment (e.g., Under wood et al, 1997;Jarvis et al, 2006). How ever, Oehlert et al (2012) in di cated that within a rel a tively short dis tance from the plat form mar gin there are com pletely dif fer ent pat terns in the as so ci a tion of d 13 C or ganic and d 13 C in or ganic val ues be cause bulk re cords are sourced from a va ri ety of produc ers in the shal low ma rine and pe lagic en vi ron ment. The lack of cor re la tion be tween d 13 C carb and d 13 C kerogen in black shales of the Kupferschiefer (Fig.…”

Facies development and sequence stratigraphy of the Ludfordian (Upper Silurian) deposits in the Zbruch River Valley, Podolia, western Ukraine: local facies overprint on the δ13Ccarb record of a global stable carbon isotope excursion

“…The Ludfordian age of these deposits is also supported by the presence of Daya navicula in the uppermost Grinchuk, Isakivtsy and Prygorodok Fms. (Tsegelnyuk et al 1983;Nikiforova et al 1985) and of Homoeospira baylei in the last two formations .…”

“…Lithostratigraphy Two main lithostratigraphical schemes exist for the Silurian of Podolia: one developed by the research group of Nikiforova (Nikiforova and Predtechenskij 1968;Nikiforova et al 1972), and the second by Tsegelnyuk (Tsegelnyuk 1980a(Tsegelnyuk , 1980b and Tsegelnyuk et al (1983). Herein the latter scheme is followed, as it corresponds more closely with our own observations on the facies development within the studied interval.…”