Organic Carbon Stable Isotope Ratios Research Articles

Description of a global marine particulate organic carbon-13 isotope data set

Abstract. Marine particulate organic carbon stable isotope ratios (δ13CPOC) provide insights into understanding carbon cycling through the atmosphere, ocean and biosphere. They have for example been used to trace the input of anthropogenic carbon in the marine ecosystem due to the distinct isotopically light signature of anthropogenic emissions. However, δ13CPOC is also significantly altered during photosynthesis by phytoplankton, which complicates its interpretation. For such purposes, robust spatio-temporal coverage of δ13CPOC observations is essential. We collected all such available data sets and merged and homogenized them to provide the largest available marine δ13CPOC data set (https://doi.org/10.1594/PANGAEA.929931; Verwega et al., 2021). The data set consists of 4732 data points covering all major ocean basins beginning in the 1960s. We describe the compiled raw data, compare different observational methods, and provide key insights in the temporal and spatial distribution that is consistent with previously observed large-scale patterns. The main different sample collection methods (bottle, intake, net, trap) are generally consistent with each other when comparing within regions. An analysis of 1990s median δ13CPOC values in a meridional section across the best-covered Atlantic Ocean shows relatively high values (≥-22 ‰) in the low latitudes (<30∘) trending towards lower values in the Arctic Ocean (∼-24 ‰) and Southern Ocean (≤-28 ‰). The temporal trend since the 1960s shows a decrease in the median δ13CPOC by more than 3 ‰ in all basins except for the Southern Ocean, which shows a weaker trend but contains relatively poor multi-decadal coverage.

Sedimentary responses to Holocene sea-level change in a shallow marine environment of southern China

This study has examined how the sedimentary processes in the prevalent non-deltaic coastal marine environments along the southeast coast of China have responded to sea-level change and other environmental drivers during the Holocene. Three sediment cores collected from a shallow marine embayment of the Lantau Island, Hong Kong, on the northern coast of the South China Sea, were analyzed. The results of particle size, total organic carbon content, organic carbon and nitrogen ratios, organic carbon stable isotope ratios, abundance of alkenones and radiocarbon dating of these cores have helped reconstruct the sedimentary history of this site. This history shares the similarity in sedimentary processes within the deltaic environment for the early Holocene, because palaeo-incised channels exist in both environments. The first 1500 years of deposition involved rapid channel infill caused by the rapidly rising sea level and strong monsoon-driven discharge. This was followed by a period of decelerated sedimentation under strong marine influence as the sea flooded the wide accommodation space in the subsequent two millennia. Major differences between the two environmental settings occur in the past 7000 years: vertical accretion was seen in the non-deltaic coastal environment, whilst lateral shoreline advance was the dominant process in the deltaic basin. The progressively slowing down in sedimentation rate in the non-deltaic site during the middle and late Holocene was caused by the reduced monsoon discharge, intensified disturbance from tropical cyclone activity and coastal currents.

Assessing the influence of physical, geochemical and biological factors on anaerobic microbial primary productivity within hydrothermal vent chimneys

Chemosynthetic primary production supports hydrothermal vent ecosystems, but the extent of that productivity and its governing factors have not been well constrained. To better understand anaerobic primary production within massive vent deposits, we conducted a series of incubations at 4, 25, 50 and 90°C using aggregates recovered from hydrothermal vent structures. We documented in situ geochemistry, measured autochthonous organic carbon stable isotope ratios and assessed microbial community composition and functional gene abundances in three hydrothermal vent chimney structures from Middle Valley on the Juan de Fuca Ridge. Carbon fixation rates were greatest at lower temperatures and were comparable among chimneys. Stable isotope ratios of autochthonous organic carbon were consistent with the Calvin-Benson-Bassham cycle being the predominant mode of carbon fixation for all three chimneys. Chimneys exhibited marked differences in vent fluid geochemistry and microbial community composition, with structures being differentially dominated by gamma (γ) or epsilon (ε) proteobacteria. Similarly, qPCR analyses of functional genes representing different carbon fixation pathways showed striking differences in gene abundance among chimney structures. Carbon fixation rates showed no obvious correlation with observed in situ vent fluid geochemistry, community composition or functional gene abundance. Together, these data reveal that (i) net anaerobic carbon fixation rates among these chimneys are elevated at lower temperatures, (ii) clear differences in community composition and gene abundance exist among chimney structures, and (iii) tremendous spatial heterogeneity within these environments likely confounds efforts to relate the observed rates to in situ microbial and geochemical factors. We also posit that microbes typically thought to be mesophiles are likely active and growing at cooler temperatures, and that their activity at these temperatures comprises the majority of endolithic anaerobic primary production in hydrothermal vent chimneys.

Changes in organic carbon stable isotope ratios across the K/T boundary: global or local control?

A global shift in carbonate carbon δ 13C-values from heavier values in the Maastrichtian to lighter values in the early Danian indicates recycling of isotopically light organic carbon to inorganic carbon reservoirs during a period of depressed marine productivity. Comparison of organic carbon δ 13-values from globally dispersed K/T sections does not show a similar, globally well-developed pattern. Several factors evidently overwhelm the potential impact of an isotopically lighter inorganic carbon source on organic matter isotopic signatures: (1) species changes in biological assemblages may modify the averaged isotopic fractionation of organic matter; and (2) shifts in the proportion of land/marine organic matter contributions to coastal marine locations may overprint the isotopic record. Local phenomena evidently outweigh global change in determining the isotope signature of organic carbon deposited in K/T boundary sections.

Soil Carbon Isotope Evidence for Holocene Habitat Change in the Kenya Rift Valley

In eastern Africa the altitude of the boundary between montane forest and lowland savanna grassland changed substantially in response to climate change during the later Holocene, but this is not clearly reflected in regional pollen records. The carbon-13 to carbon-12 ratios of tropical grasses are higher than those of most other plants, and this difference is preserved in soil organic carbon stable isotope ratios. Soil organic matter (13)C/(12)C ratios in profiles along an altitude transect in the central Rift Valley of Kenya suggest that the forest-savanna boundary advanced more than 300 meters in altitude. This could have implications for understanding the effects of climate change on the configuration of floral zones, prehistoric hunter-gatherer land-use patterns, and the timing of the advent of Neolithic food production.