Beschreibung: Seep carbonates provide excellent records of past seepage activities, and have been commonly considered to preserve primary, unaltered stable carbon isotope signatures. However, late diagenetic reactions may overprint original isotopic compositions, but the mode and effect of such alterations are poorly understood. In particular, there are significant uncertainties regarding how carbon and strontium isotopic compositions of seep carbonates respond to diagenesis. This study reports recently discovered Cretaceous hydrocarbon-seep deposits from the Yarlung-Zangbo Suture Zone, Tibet, China that have experienced substantial diagenetic alteration that is shown by recrystallization and secondary veins. Unitary linear recursive analysis was applied to d13C values and 87Sr/86Sr ratios of the seep carbonates to evaluate the degree of secondary modification and to quantitatively constrain the compositions of primary carbonates and late diagenetic fluids. The d18O values range from - 11.8 per mil to - 2.2 per mil, d13C values from - 34.1 per mil to - 12.9 per mil and 87Sr/86Sr ratios from 0.706221 to 0.706808. The heterogeneity in isotopic compositions and the observation that the most negative d18O values occur in samples with the most extensive recrystallization indicate significant and spatially heterogeneous modification of isotope compositions during late diagenesis. The linear correlations between d13C values and d18O values for matrix micrites (R2 = 0.54), and between bulk carbonate 87Sr/86Sr ratios and d18O values (R2 = 0.85) are best explained by burial diagenetic overprinting of oxygen, strontium, and even carbon isotopic compositions rather than by meteoric water hypergenesis. Extrapolated values of d13C and ratios of 87Sr/86Sr against a d18O value of - 2 per mil (average value of calcite precipitated in isotopic equilibrium with coeval Cretaceous seawater) that would characterize the primary carbonate, give an end member d13C value of - 34 per mil and an end member 87Sr/86Sr ratio of 0.7072. The end member isotopic values obtained by this extrapolation suggest that the primary seep carbonates with low d13C values and high 87Sr/86Sr ratios were formed by anaerobic oxidation of methane near the seafloor. In contrast, the measured d18O values and 87Sr/86Sr ratios reflect late diagenetic fluids represented by burial pore water characterized by a low 87Sr/86Sr ratio and high temperature. Our findings reveal that d13C values can only be moderately and 87Sr/86Sr ratios can be significantly altered during late diagenesis, and show that it is possible to quantitatively assess the primary composition of diagenetically altered seep carbonates.

Beschreibung: Authigenic carbonates and pyrite associated with sulfate-driven anaerobic oxidation of methane (AOM) at methane seeps provide archives to explore the biogeochemical processes involved and seepage dynamics over time. The wide range and extremely high δ34S values of pyrite (δ34Spy) have been used to trace the AOM-related processes. However, the detailed mechanism for this phenomenon is not well understood. We propose that the characteristics of δ34Spy were mainly controlled by the competition between sulfate reduction and sulfate supply, as well as the redox condition. To test this hypothesis, we investigated Sr/Ca and Mg/Ca ratios, trace element compositions, pyrite contents and sulfur isotopic compositions in seep carbonates from Site F and Haima in the northern South China Sea. Calcite and aragonite contents were distinguished through the Sr/Ca and Mg/Ca ratios. The data show that aragonites are always associated with relatively low δ34Spy values compared to calcites. The Mo contents show a good correlation with pyrite contents in calcites and aragonites, and the slope in aragonites is larger than that in calcites. This relationship indicates that the aragonite precipitated in a relatively open system with higher Mo availability. Thus, we conclude that sulfides with low δ34S values formed at high supply of sulfate under the relatively open system with respect to diffusive replenishment of sulfate, where the carbonate precipitation occurred close to the seafloor due to a strong methane flux. Under vigorous methane flux simultaneously, the high potential of less anoxic conditions, which could limit the additional pyrite accumulation and/or favor the microbial disproportionation, could also be the cause of the low δ34Spy, as supported by samples from the Haima sites. Evidence for this assumption is based on the occurrence of bivalve shells and less enrichment in As and Sb. Conversely, the positive δ34Spy values result from near to complete exhaustion of dissolve sulfate via AOM within a deeper sulfate-methane transition zone, where Mo is less available. The combination of a detailed elemental study of authigenic carbonates with sulfur isotopic composition of sulfide minerals in carbonates is a promising tool for reconstructing the dynamics of seep intensities at modern and, potentially, geological seep sites.

Beschreibung: Highlights • Elemental and radiocarbon analysis of southern Mariana Trench (SMT) sediments since 30 ka. • Shift to more reducing (suboxic) deep waters during the Last Glacial Maximum (LGM). • Blooms of giant diatom Ethmodiscus rex formed laminated diatom mats (LDMs) during LGM. •Redox changes were due to increased primary productivity induced by enhanced Asian dust inputs. • Biogenic and authigenic apatite are the main carriers of rare earth elements (REEs) in SMT sediments. Abstract: The modern southern Mariana Trench is characterized by oligotrophic surface waters, resulting in low primary productivity and well-oxygenated bottom waters. This study investigates changes in the redox conditions of bottom waters in the southern Mariana Trench during the Last Glacial Maximum (LGM) and their potential causes. We measured major, trace, and rare earth elements (REE) in three gravity cores (GC03, GC04, and GC05) and one box core (BC11) retrieved from the southern Challenger Deep at water depths from 5289 to 7118 m. The upper sediment layers of both GC05 and BC11 are dominated by valve fragments of the giant diatom Ethmodiscus rex, forming laminated diatom mats (LDMs). 14C-AMS dates of bulk organic matter show that the LDMs accumulated between 18.4 and 21.8 kyr B.P., corresponding to the LGM. Modest enrichments of U and Mo along with weak or absent Ce anomalies in the LDM point to suboxic conditions during the LGM. In contrast, non-LDM samples exhibit little to no enrichment of redox-sensitive elements as well as negative Ce anomalies, indicating deposition under oxic bottom-water conditions. The Ce anomalies are considered valid proxies for bottom-water redox conditions because REE signatures were acquired in the early diagenetic environment, as indicated by strong P-REE correlations and middle-REE enrichment associated with early diagenetic cycling of Fe-Mn oxyhydroxides in the sediment column followed by capture of the REE signal by biogenic and/or authigenic apatite. We postulate that the more reducing bottom-water conditions during the LGM were linked to increased primary productivity induced by enhanced Asian dust input. As shown in earlier studies, the increased primary productivity associated with Ethmodiscus rex blooms in the eastern Philippine Sea played a significant role in capturing atmospheric CO2 during the LGM. Consequently, the magnitude of atmospheric CO2 sequestration by giant diatom blooms during the LGM may have been greater than previously envisaged.