Radiocarbon and Luminescence Dating of Lacustrine Sediments in Zhari Namco, Southern Tibetan Plateau

There are more than 1,000 lakes within the Tibetan Plateau (TP), all of which are sensitive to changes in regional climate and local hydrology. Lacustrine sediments within these lakes preserve a good record of these changes. However, determining their precise ages is difficult due to the complex nature of lake reservoir effects (LRE), which limit our understanding of paleoenvironmental changes. Focusing on an exposed 600 cm thick lacustrine sediment profile located in western Zhari Namco, we used a combination of both radiocarbon and optically stimulated luminescence (OSL) dating methods in order to evaluate the carbon reservoirs of bulk organic matter (BOM) and aquatic plant remnants (APR), and to explore the age differences between 14C and OSL and their respective reliability. We demonstrated that (i) OSL ages were changed in stratigraphic order, and the OSL age just below the beach gravel layer was consistent with previously reported paleoshoreline ages; (ii) 14C ages were divergent between BOM and grass leaves; (iii) 14C ages of BOM were older than 14C ages of APR; and (iv) all 14C ages were older than OSL ages. This could be attributed to changing LRE in the past, causing the 14C ages to appear unstable during the deposition period. Although the 14C ages of terrestrial plant remnants (TPR) were not affected by LRE, an analyzed twig nonetheless returned a 14C age older than its respective layer’s OSL age, suggesting it may have been preserved on land prior to transportation into the lake. Our study suggests that OSL ages are more reliable than 14C ages with respect to Zhari Namco lacustrine sediments. We recommend caution when interpreting paleoenvironmental changes based on lacustrine sediment 14C ages alone.

Habitat Partitioning in the Marine Sector of Karst Subterranean Estuaries and Bermuda's Marine Caves: Benthic Foraminiferal Evidence

Karst subterranean estuaries (KSEs) are created from the two- and three-way mixing of saline groundwater, rain, and oceanic water in the subsurface on carbonate landscapes, and this hydrographic framework promotes unique physical processes, biogeochemical cycling, and biological communities. Here we provide evidence that the source and quantity of particulate organic matter (POM) that is delivered to the benthos strongly correlates to benthic habitat partitioning in the oxygenated marine sectors of KSEs. A dataset of benthic foraminifera at 128 different locations from several large flooded cave systems in Bermuda were compiled and evaluated against common environmental characteristics (e.g., tidal exposure, substrate particle size, bulk organic matter, C:N, total organic carbon, and δ13Corg). Benthic areas receiving more carbon isotopically depleted organic matter sources (mean δ13Corg values < −23.2‰, C:N ratios >11), most likely from the terrestrial surface and some marine plankton, were dominated by Trochammina inflata, Bolivina spp., and Helenina anderseni. In contrast, benthic areas receiving more carbon isotopically enriched organic matter sources (mean δ13Corg values > −21.6‰, C:N ratios <10), most likely from marine plankton transported through marine cave openings cave from adjacent coastal waters, were dominated by Spirophthalmidium emaciatum, Spirillina vivipara, Patellina corrugata, and Rotaliella arctica. The benthic foraminifera most distal from any cave entrances were dominated by taxa also known from the deep-sea (e.g., Rotaliella, Spirophthalmidium) in sediment with the lowest bulk organic matter content (mean: 6%), or taxa that prefer hard substrates and are potentially living attached to cave walls (Patellina, Spirillina). While physical groundwater characteristics (e.g., salinity, dissolved oxygen) are expected drivers of benthic ecosystems in KSEs, these results suggest that POM source, quantity, and delivery mechanisms (e.g., groundwater-seawater circulation mechanisms, terrestrial flux) play an important role in benthic habitat partitioning and the spatial variability of biogeochemical cycles in the oxygenated marine sector of KSEs.

Warming and elevated CO2 promote incorporation of plant-derived lipids into soil organic matter in a spruce-dominated ombrotrophic bog

<p>More than one third of global soil organic matter (SOM) is stored in peatlands, despite them occupying less than 3% of the land surface. Increasing global temperatures have the potential to stimulate the decomposition of carbon stored in peatlands, contributing to the release of disproportionate amounts of greenhouse gases to the atmosphere but increasing atmospheric CO<sub>2</sub> concentrations may stimulate photosynthesis and return C into ecosystems.  Key questions remain about the magnitude and rate of these interacting and opposite processes to environmental change drivers.</p><p>We assessed the impact of a 0–9°C temperature gradient of deep peat warming (4 years of warming; 0-200 cm depth) in ambient or elevated CO<sub>2</sub> (2 years of +500 ppm CO<sub>2</sub> addition) on the quantity and quality of SOM at the climate change manipulation experiment SPRUCE (Spruce and Peatland Responses Under Changing Environments) in Minnesota USA. We assessed how warming and elevated CO<sub>2</sub> affect the degradation of plant and microbial residues as well as the incorporation of these compounds into SOM. Specifically, we combined the analyses of free extractable <em>n</em>-alkanes and fatty acids together with measurements of compound-specific stable carbon isotopes (δ<sup>13</sup>C).</p><p>We observed a 6‰ offset in δ<sup>13</sup>C between bulk SOM and <em>n</em>-alkanes, which were uniformly depleted in δ<sup>13</sup>C when compared to bulk organic matter. Such an offset between SOM and <em>n</em>-alkanes is common due to biosynthetic isotope fractionation processes and confirms previous findings. After 4 years of deep peat warming, and 2 years of elevated CO<sub>2</sub> addition a strong depth-specific response became visible with changes in SOM quantity and quality. In the upper 0-30 cm depth, individual <em>n</em>-alkanes and fatty acid concentrations declined with increasing temperatures with warming treatments, but not below 50 cm depth. In turn, the δ<sup>13</sup>C values of bulk organic matter and of individual <em>n</em>-alkanes and fatty acids increased in the upper 0-30 cm with increasing temperatures, but not below 50 cm depth. Thus <em>n</em>-alkanes, which typically turnover slower than bulk SOM, underwent a rapid transformation after a relatively short period of simulated warming in the acrotelm. Our results suggest that warming accelerated microbial decomposition of plant-derived lipids, leaving behind more degraded organic matter. The non-uniform, and depth dependent warming response implies that warming will have cascading effects on SOM decomposition in the acrotelm in peatlands. It remains to be seen how fast the catotelm will respond to rising temperatures and atmospheric CO<sub>2</sub> concentrations.</p>

Homeostatic response of Aptian gymnosperms to changes in atmospheric CO2 concentrations

The sensitivity of plant carbon isotope fractionation (13Δleaf) to changes in atmospheric CO2 concentrations (Ca) is the subject of heavy debate, with some studies finding no sensitivity, while others show a strong dependency. We tested the hypothesis of photosynthetic homeostasis by using δ13C of n-alkanes, cuticles, and bulk organic matter of gymnosperm-rich rocks (Arundel Clay) from two sites deposited during the Aptian, a time that experienced significant Ca variations. Our results show no effect of Ca on 13Δleaf, and a relatively constant Ci/Ca (0.64 ± 0.04, 1σ; i—intercellular space), a value that is similar to that of modern gymnosperms. These results suggest that Aptian gymnosperms used homeostatic adjustments with rising Ca, probably involving increased carbon assimilation and/or stomatal closure, a response also found in modern gymnosperms. The similarity between Aptian and modern gymnosperms suggests that the processes responsible for regulating CO2 and water vapor exchange during photosynthesis have remained unaltered in gymnosperms for the past 128 m.y.

Late Pleistocene environments of the Bighorn Basin, Wyoming-Montana, USA

Limited numbers of high-resolution records predate the Last Glacial Maximum (LGM) making it difficult to quantify the impacts of environmental changes prior to peak glaciation. We examined sediments from Last Canyon Cave in the Pryor Mountains of Montana and Wyoming to construct a >45 ka environmental record from pollen and stable isotope analysis. Artemisia pollen was hyper-abundant at the beginning of the record. Carbon isotope values of bulk organic matter (>40 ka) showed little variation (-25.3 ± 0.4‰) and were consistent with a arid C3 environment, similar to today. After 40 cal ka BP, Artemisia pollen decreased as herbaceous taxa increased toward the LGM. A significant decrease in δ13C values from 40–30 cal ka BP (~1.0‰) established a new baseline (-26.6 ± 0.2‰), suggesting cooler, seasonally wetter conditions prior to the LGM. These conditions persisted until variation in δ13C values increased significantly with post-glacial warming, marked by two spikes in values at 14.4 (-25.2‰) and 13.5 cal ka BP (-25.4‰) before δ13C values dropped to their lowest values (-26.9 ± 0.2‰) at the onset of the Younger Dryas (12.8 ka). These results provide insights into late Pleistocene conditions and ecological change in arid intermontane basins of the Rocky Mountains.

Seasonal deposition processes and chronology of a varved Holocene lake sediment record from Chatyr Kol lake (Kyrgyz Republic)

Microfacies analysis of a sediment record from Chatyr Kol lake (Kyrgyz Republic) reveals the presence of seasonal laminae (varves) from the sediment base dated at 11 619±603 BP (years Before Present) up to ∼360±40 BP. The Chatvd19 floating varve chronology relies on replicate varve counts on overlapping petrographic thin sections with an uncertainty of ±5 %. The uppermost non-varved interval was chronologically constrained by 210Pb and 137Cs gamma spectrometry and interpolation based on varve thickness measurements of adjacent varved intervals with an assumed maximum uncertainty of 10 %. Six varve types were distinguished, are described in detail, and show a changing predominance of clastic-organic, clastic-calcitic or clastic-aragonitic, calcitic-clastic, organic-clastic, and clastic-diatom varves throughout the Holocene. Variations in varve thickness and the number and composition of seasonal sublayers are attributed to (1) changes in the amount of summer or winter/spring precipitation affecting local runoff and erosion and/or to (2) evaporative conditions during summer. Radiocarbon dating of bulk organic matter, daphnia remains, aquatic plant remains, and Ruppia maritima seeds reveals reservoir ages with a clear decreasing trend up core from ∼6150 years in the early Holocene, to ∼3000 years in the mid-Holocene, to ∼1000 years and less in the late Holocene and modern times. In contrast, two radiocarbon dates from terrestrial plant remains are in good agreement with the varve-based chronology.

Biosignatures Associated with Freshwater Microbialites

Freshwater microbialites (i.e., lithifying microbial mats) are quite rare in northern latitudes of the North American continent, with two lakes (Pavilion and Kelly Lakes) of southeastern BC containing a morphological variety of such structures. We investigated Kelly Lake microbialites using carbon isotope systematics, phospholipid fatty acids (PLFAs) and quantitative PCR to obtain biosignatures associated with microbial metabolism. δ13CDIC values (mean δ13CDIC −4.9 ± 1.1‰, n = 8) were not in isotopic equilibrium with the atmosphere; however, they do indicate 13C-depleted inorganic carbon into Kelly Lake. The values of carbonates on microbialite surfaces (δ13C) fell within the range predicted for equilibrium precipitation from ambient lake water δ13CDIC (−2.2 to −5.3‰). Deep microbialites (26 m) had an enriched δ13Ccarb value of −0.3 ± 0.5‰, which is a signature of photoautotrophy. The deeper microbialites (>20 m) had higher biomass estimates (via PLFAs), and a greater relative abundance of cyanobacteria (measured by 16S copies via qPCR). The majority of PLFAs constituted monounsaturated and saturated PLFAs, which is consistent with gram-negative bacteria, including cyanobacteria. The central PLFA δ13C values were highly depleted (−9.3 to −15.7‰) relative to δ13C values of bulk organic matter, suggesting a predominance of photoautotrophy. A heterotrophic signature was also detected via the depleted iso- and anteiso-15:0 lipids (−3.2 to −5.2‰). Based on our carbonate isotopic biosignatures, PLFA, and qPCR measurements, photoautotrophy is enriched in the microbialites of Kelly Lake. This photoautotrophy enrichment is consistent with the microbialites of neighboring Pavilion Lake. This indication of photoautotrophy within Kelly Lake at its deepest depths raises new insights into the limits of measurable carbonate isotopic biosignatures under light and nutrient limitations.

Evidence for repeated and global palaeoenvironmental perturbations during the Aalenian

<p>Over the last decades, studies on Jurassic palaeoenvironments have been mostly focussed on the early Toarcian as this latter was marked by the Toarcian Oceanic Anoxic Event (T-OAE; ca. 183 Ma), which was one of the most extreme hyperthermal of the Phanerozoic. Hence, little is know about palaeoclimatic and palaeoenvironmental changes during the Aalenian time interval, although it is likely marked by an abrupt cooling in the aftermath of the Toarcian warm mode. Available palaeontological and geochemical datasets suggest that the Aalenian is also characterized by faunal turnovers and potential carbon-cycle perturbations. Despite those evidence, there is still no consensus about the modality of Aalenian palaeoenvironmental and palaeoclimatic changes as well as their potential triggering mechanisms. In addition, data from outside Europe are absent, leading to large uncertainties whether the observed changes are of gobal significance. In this study, we focus on the upper Toarcian–lower Bajocian interval of two marl/limestone alternation successions, namely Le Brusquet (Vocontian Basin, SE France) and El Peñon (Andean Basin, N Chile). Palaeoenvironmental and palaeoclimatic conditions are inferred based on high-resolution mineralogical (whole-rock and clay fraction) and geochemical (carbon isotopes, Rock-Eval pyrolysis, phosphorus, mercury) analyses. Additionaly, we provide a cyclostratigraphic framework for the Aalenian based on high-resolution magnetic susceptibility spectral analysis. The carbon isotope composition of bulk organic matter reveals evident correlatable fluctuations between sites from both hemispheres, providing the first evidence that the carbon cycle was globally and repeatedely disturbed during the Aalenian. The Toarcian–Aalenian transition is associated with a decrease in detrital and nutrient input (phosphorus), which is likely related to the shift towards the Aalenian cool mode. Interestingly, the middle–upper Aalenian transition is characterized by a sharp increase in terrigenous and nutrient influxes suggesting a more humid and warmer episode. The concomitance between strongly expressed precession cycles and palaeoenvironmental changes suggests moreover the influence of orbital parameters on the Aalenian sedimentary record.</p>