Effects of early marine diagenesis and site-specific depositional controls on carbonate-associated sulfate : insights from paired S and O isotopic analyses

Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund (#57548-ND2) to D.F. for partial support of this research and from the Estonian Research Council (#PUT611, #PRG836) to O.H and A.L.Carbon, sulfur and oxygen isotope profiles in Silurian strata of the Baltoscandian Basin (Estonia), coincident with the Ireviken Bioevent, provide insights into basin-scale and platform-specific depositional processes. Paired carbon isotope records preserve a positive isotope excursion during the early Wenlock, coincident with faunal turnover, yet δ13C variability of this excursion compared to other locations within the paleobasin reflects local depositional influences superimposed on a global signal. In comparison, sulfur isotope records do not preserve a systematic isotopic excursion over the same interval. Instead, sulfur isotope records have high sample-to-sample stratigraphic variability, particularly in shallow-water carbonate rocks (scatter up to ~10‰ for δ34SCAS and ~ 25‰ for δ34Spyr). This pattern of isotopic variability is also found between sites from the same carbonate platform, where the magnitude and isotopic variability in δ34SCAS and δ34Spyr differ depending on relative local sea level (and therefore facies). Such facies-dependent variability reflects more closed- versus more open-system diagenetic conditions where pulses of increased sedimentation rate in the shallow water environments generates greater isotopic variability in both δ34SCAS and δ34Spyr. Increased reworking and proximity to the shoreline results in local sulfide oxidation, seen as a decrease in δ34SCAS in the most proximal settings. Platform-scale evolution of isotopically distilled pore-fluids associated with dolomitization results in increased δ34SCAS in deep water settings. Correlations in paired δ34SCAS-δ18OCAS data support these conclusions, demonstrating the local alteration of CAS during deposition and early marine diagenesis. We present a framework to assess the sequence of diagenetic and depositional environmental processes that have altered δ34SCAS and find that δ34S of ~27–28‰ approximates Silurian seawater sulfate. Our findings provide a mechanism to understand the elevated variability in many deep-time δ34SCAS records that cannot otherwise be reconciled with behavior of the marine sulfate reservoir.PostprintPeer reviewe

Linking the progressive expansion of reducing conditions to a stepwise mass extinction event in the late Silurian oceans

The late Ludlow Lau Event was a severe biotic crisis in the Silurian, characterized by resurgent microbial facies and faunal turnover rates otherwise only documented during the "big five" mass extinctions. This asynchronous late Silurian marine extinction event preceded an associated positive carbon isotope excursion (CIE), the Lau CIE, although a mechanism for this temporal offset remains poorly constrained. Here, we report thallium isotope data from locally reducing late Ludlow strata within the Baltic Basin to document the earliest onset of global marine deoxygenation. The initial expansion of anoxia coincided with the onset of the extinction and therefore preceded the Lau CIE. Additionally, sulfur isotope data record a large positive excursion parallel to the Lau CIE, interpreted to indicate an increase in pyrite burial associated with the widely documented CIE. This suggests a possible global expansion of euxinia (anoxic and sulfidic water column) following deoxygenation. These data are the most direct proxy evidence of paleoredox conditions linking the known extinction to the Lau CIE through the progressive expansion of anoxia, and most likely euxinia, across portions of the late Silurian oceans

Internal stratification of two thick Ordovician bentonites of Estonia: deciphering primary magmatic, sedimentary, environmental and diagenetic signatures

Twenty-six samples from two major altered volcanic ash beds, Kinnekulle and BII Bentonite of the Kuressaare core section (K-3), Saaremaa Island, were explored to record the geochemical and mineralogical heterogeneity of beds. Signs of ash transport fractionation, redeposition of volcanic ash and diagenetic redistribution of material are described and interpreted. In authigenic mineralogy of the Kinnekulle Bentonite illite–smectite dominates with addition of K-feldspar at the margins. The BII Bentonite is composed of chlorite–smectite and illite–smectite. The stability of phenocryst compositions, including that of sanidine and biotite, indicates that both bentonites originate from a single eruption. The observed rather stable pyroclastic sanidine compositions in the cross section of bentonites confirm the reliability of sanidine-based fingerprinting of altered volcanic ash beds. Trace element distribution in bentonites and host rocks indicates that Zr, Ga, Rb, Nb, Ti and Th stayed largely immobile during volcanic ash alteration and reflect primary ash composition. However, some redistribution of Nb and Ti as well as Y has probably occurred near the contacts of bentonite with the host rock. More scattered grain size distribution and immobile element patterns of the Kinnekulle Bentonite support the idea that the primary ash bed had a heterogeneous composition and it was one of the biggest bentonites of the Phanerozoic and most likely records an extended volcanic event. Significant geochemical variations, including a high S content, near the upper and lower contacts of the Kinnekulle Bentonite and elevated Ca and P in host rocks of both bentonites suggest that the studied large ash-falls caused notable perturbations in shallow marine and early post-sedimentary environment

Reconstructing the environmental conditions around the Silurian Ireviken Event using the carbon isotope composition of bulk and palynomorph organic matter

The carbon isotope composition (δ13C) of bulk organic matter and two palynomorph groups (scolecodonts and chitinozoans) from the Llandovery-Wenlock strata of Gotland (E Sweden) are compared to gain knowledge about carbon cycling in the Silurian (sub)tropical shelf environment. The δ13C values of the palynomorphs are mostly lower than the δ13C values of the bulk organic matter, and the δ13C values of the benthic scolecodonts are lower than those of the planktonic chitinozoans. While the difference between bulk and palynomorph δ13C may be in part a function of trophic state, the lower values of the scolecodonts relative to those of chitinozoans, which are assumed to live in the well-mixed water column, might imply an infaunal mode of life for the polychaetes that carried the scolecodonts. Lower δ13C for the scolecodonts in the middle of the section may represent variations in primary marine productivity (supported by acritarch abundance data), oxidation of organic matter in the bottom waters, or genera effects. In general, however, trends between the three data sets are parallel, indicating similarities in the low frequency, environmentally forced controls. The δ13C data show a decreasing trend from the base of the section, up to a horizon well below the base of the Upper Visby Formation. At this level, and therefore probably several 10 kyr before the δ13C increase in the carbonates, the δ13C organic values increase by ~1‰. This perhaps is an expression of a changed composition of the bulk organic matter associated with the extinction events prior to the Llandovery-Wenlock boundary