Component-specific carbon and oxygen isotope variability in marine limestones of the Mid-Ludfordian (late Silurian) carbon isotope excursion of the Baltic Basin

Abstract

The Vidukle-61 core, located in the Baltic Basin of central Lithuania, contains an excellent record of the Mid-Ludfordian Carbon Isotope Excursion (MLCIE; c. 1 Myr duration) identified by carbonate δ¹³C values rising steadily from c. 0 ‰ to 8 ‰, followed by a return to c. 0 ‰ through a 40-m thick interval of marine limestone. Here, we report carbon and oxygen (CO) isotope data obtained by secondary ion mass spectrometry (SIMS) on two samples (VID-1 and VID-3) of bioclastic packstone from the zenith of the MLCIE. As reported in a previous study, the two samples have bulk-rock values of 7.37 ‰ and 8.17 ‰ for δ¹³C and − 5.27 ‰ and − 4.68 ‰ for δ¹⁸O (values relative to VPDB and repeatability ±0.1 ‰). In contrast, high-spatial-resolution SIMS analyses of the same samples show δ¹³C values for the calcitic matrix and spar cement ranging between c. 5 ‰ and 11 ‰, and for fossil fragments between c. 6 ‰ and 13 ‰. δ¹⁸O SIMS values of individual components vary by up to ±2 ‰ relative to the bulk-rock baseline values. Based on SIMS results and observed isotopic variations between individual components, we attribute the bulk-rock values as an admixture of original seawater CO isotope compositions that are best preserved in the bioclasts and the isotopic compositions of later-stage fluids from which spar and matrix-bound microspar precipitated. Our findings emphasize the significant differences between component-specific and bulk-rock CO isotope values. While δ¹³C_bulk curves may capture broad temporal trends in the carbon cycle, high-spatial-resolution and component-specific data reveal microscale isotopic heterogeneity, which can deviate considerably from bulk-rock values. Differentiating and identifying the values that best record original carbonate CO isotopic compositions, even in well-preserved carbonate rocks, requires micro-scale studies to resolve C-isotope heterogeneity in carbonate rocks.