Bulk Sediment Research Articles

Changing sediment supply during glacial-interglacial intervals in the North Atlantic revealed by particle size characterization and environmental magnetism

The Pliocene-Pleistocene transition is characterized by an abundance of Ice-Rafted Debris (IRD) in the North Atlantic basin. One of the regions affected by IRD during this period is the Gardar Drift, where the DSDP Leg 94 Hole 611A is located. This region received sediments from different sources during the glacial and interglacial intervals (e.g., Iceland and Greenland). We analyzed grain size and particle-size specific magnetic properties of sediments for their provenance characterization between ~2.64 and 2.52 Ma. Our results show that major proportion of bulk sediments during both glacial and interglacial periods were made up of basaltic-rich Icelandic sediments, whereas only during more intense glacial periods (Marine Isotope Stages 100 and 104), a small proportion of non-basaltic sand compositions were identified, possibly sourced from Greenland and other non-basaltic provenances. The non-basaltic sand fractions during the intense glacial periods were likely supplied as IRDs. In addition, a new level of coarse lithics (38 pcs. of >1 mm) composed of different rocks types (e.g., basalt, granite, granodiorite etc.) were identified in DSDP 611A Hole during the end of glacial MIS 104. The coarse lithic fragments showed distinctive magnetic properties than rest of the particle sizes and were classified as Iceberg-Rafted Debris (IBRD). Our results show that higher sand percentage was found during the intense glacial episodes, and their magnetic grain size analysis could help in distinguishing their provenance. We elaborate that particle size specific magnetic measurements of sand fractions could help in rapidly characterizing the glacial episodes in the subpolar North Atlantic.

Carbon storage of Mangrove Sediments in Kibani, Eastern Niger Delta, Nigeria: Implications on the Environment

The amount of carbon stored in the mangrove sediments in Kibani area of Eastern Niger Delta was quantified. The objectives were to determine the sediment bulk density (SBD) and organic carbon concentration (%C), and estimate the amount of organic carbon stock in the top 100-cm depth of mangrove sediments and its carbon dioxide equivalent (CO2 e). A total of 312 sediment samples were collected from 24 different sampling points at seven locations. An open-cylindrical gouge auger of 100-cm length and 5-cm diameter was designed and fabricated for the purpose of sediment sampling. Field measurement, laboratory analysis, data presentation and result presentation followed the Guidelines for Forest Carbon Measurement, Monitoring and Reporting. The sediment type has a major control on the distribution of organic carbon. The lithology is a typical of mangrove forest and included peat and clay with pockets of sands. Sediment bulk density (SBD) and %C ranged from 0.95 g/cm3 to 1.97 g/cm3 (mean = 1.30 g/cm3 ) and 0.9 % to 33.6 % (mean = 12.48 %) respectively. Carbon stocks ranged from 24.12 ± 15.18 Mg C ha-1 to 3245.76 ± 15.18 Mg C ha-1 (mean value = 39.80 Mg ha-1) while CO2 e ranged 88.52 ± 15.18 Mg CO2 ha-1 to 11,911.94 ± 15.18 Mg CO2 ha-1 (mean = 2,348.07 ± 15.18 Mg CO2 ha-1). The total carbon stock of the study area was estimated at 947.78 ± 55.64 Gg C, equivalent to 3,478.36 ± 55.64 Gg CO2 . The study revealed that 732,595.71 ± 55.64 Mg of carbon dioxide is stored in the sediments. This amount could either be removed or released to the atmosphere through forest management practices and/or degradation to exacerbate climate change and /or mitigate it. It is recommended that Niger Delta region of Nigeria should be protected against all anthropogenic-related degradation which influence the amount of carbon storage.

Contemporary Controls on Terrestrial Carbon Characteristics in Temperate and Sub‐Tropical Australian Wetlands

AbstractA thorough understanding of controls over terrestrial sedimentary organic carbon characteristics in both the present and the past is pivotal to better understand atmospheric CO2 pathways into depositional sinks such as peats, swamps, and lakes. We explored the relationship between wetland sediment organic matter storage, climate (precipitation, temperature) and catchment vegetation data (catchment vegetation cover in percent; leaf carbon content in g/m2) by means of multivariate statistical analyses to investigate patterns of carbon deposition in modern wetlands and to provide a more robust framework for interpreting sediment bulk organic geochemistry as a proxy for past carbon cycling. Carbon and nitrogen elemental concentration and stable isotope composition were analyzed from sub‐surface sediments at 18 wetlands in eastern Australia. The statistical analyses indicate that variability in geochemical organic matter data in wetland sediments is best explained by geographic differences in catchment vegetation cover and, by inference, the balance of terrestrial versus aquatic organic matter input to the sediment. TOC/TN of aquatic matter may be additionally driven toward higher (terrestrial) values by nitrogen limitation in the catchment and the lakes. These processes explain up to ∼40% of the total variance in the sediment geochemistry (redundancy analyses). Up to ∼10% of the total variance may be attributed to post‐depositional processes and organic matter remineralization. The remaining ∼50% of total variance in the data may be attributed to local conditions across the sites, geochemical processes that were not captured in this study, or to the different timescales covered by the sediments at each site.

A chronologically reliable record of 17,000 years of biomass burning in the Lake Victoria area

Fire regimes differ across tropical and subtropical biomes depending on multiple parameters whose interactions and levels of importance are poorly understood, particularly at multidecadal and longer time scales. In the catchment of Lake Victoria, savanna, rainforest, and Afromontane vegetation have interspersed over the last 17,000 years, which may have influenced the fire regime and vice versa. However, climate and humans are most often the primary drivers of fire regime changes, and analysing their respective roles is critical for understanding current and future fire regimes. Besides a handful of radiocarbon dates on grassy charcoal, the timescales of published studies of Lake Victoria sediment chronologies rely mostly on dates of bulk sediment, and chronological disagreements persist, mainly due to variation between estimations of the 14C reservoir effect. Here, we provide independent 14C chronologies for three Late Glacial and Holocene lacustrine sediment cores from various water depths and compare them with the biostratigraphy to establish a new chronological framework. We present the first continuous sedimentary charcoal records from Lake Victoria; these suggest that fire activity varied substantially during the past 17,000 years. Our new pollen records reveal the long-term vegetation dynamics. The available evidence suggests that before human impact increased during the Iron Age (ca. 2400 yr BP), biomass burning was linked to climate and vegetation reorganizations, such as warming, drying, and the expansion of rainforests and savannas. Our results imply that climate can trigger substantial fire regime changes and that vegetation responses to climate change can co-determine the fire regime. For instance, biomass burning decreased significantly when the rainforest expanded in response to increasing temperatures and moisture availability. Such insights into the long-term linkages between climate, vegetation, and the fire regime may help to refine ecosystem management and conservation strategies in a changing global climate.

Magnetic properties of surface soils in the upper and middle reaches of the Yarlung Zangbo River Basin, southern Tibetan Plateau, and their environmental significance

SUMMARY The relationship between climate and the magnetic properties of surface soils on the Chinese Loess Plateau (CLP) have been widely used to provide the basis for palaeoclimatic reconstruction, based on the underlying loess/palaeosol sequences. To date, however, there are few investigations of variations in the magnetic properties of surface soils on the Tibetan Plateau (TP), especially on the southern TP. Therefore, it remains unclear whether magnetic properties could be used as proxies for palaeoclimatic reconstruction in the region. In this study, environmental magnetic parameters and bulk sediment grain-size measurements were made on a set of surface samples from the Yarlung Zangbo River Basin (YZRB), on the southern TP, and their environmental significance was evaluated. The results reveal spatial differences in the magnetic properties of the surface soils, likely caused by regional climatic factors. The input of primary magnetic minerals is the driver of the magnetic properties of the samples from the western (Gar-Saga) part of the YZRB, where the magnetic variations are controlled by the influence of wind intensity on the local source material. However, the samples from the eastern region (Gongga-Nyingchi) are significantly affected by the topographic and sedimentary conditions, and there is no relationship between the magnetic properties and climate. The samples from the central part of the study area (Saga-Gongga) show obvious signs of a pedogenic influence on magnetic properties and ultrafine superparamagnetic (SP) and single domain (SD) ferrimagnetic minerals dominate the magnetic susceptibility. Combined with the variation of grain size, this implies the influence of wind intensity and pedogenesis contribute to the magnetic enhancement. However, the pedogenic intensity closely related to precipitation and temperature could offer the possibility of using sedimentary magnetic properties for palaeoclimatic reconstruction. Furthermore, the existence of a distinct pedogenic alteration boundary in the Saga area is consistent with the location of the Indian summer monsoon, which may be the dominant control on the pedogenic intensity.

Thermal structure and sediment bulk density of the MV420 mud volcano on the continental slope of the Canadian Beaufort Sea and its implications for the plumbing system

The shallow migration path of mudflow of the mud volcano MV420 on the continental slope of the Canadian Beaufort Sea is investigated in terms of thermal and geotechnical characteristics. MV420 is a nearly flat topped active mud volcano that emits methane and fluidized mud. Its top is at a depth of water of 420 m, within the gas hydrate stability zone. During the summer 2017 IBRV Araon expedition, several measurements of marine heat flow were conducted in the area of juvenile mud mound/pond morphologies identified by multibeam bathymetry and backscatter intensity images obtained by the Monterey Bay Aquarium Research Institute’s autonomous underwater vehicle. The heat probe (a gravity corer with temperature, pressure, and tilt sensors) appeared to penetrate to a depth of >70 m below the seafloor. The sediment bulk density in the mudflow migration path of MV420, estimated by pressure change with controlled stretches of a winch cable, was 1.56 g/cm3. The subsurface temperature reaches up to 18°C and profiles show that the maximum local temperature occurs from 20 to 40 m below the seafloor, implying substantial vertical variability of geothermal gradients. Our finding of large positive and small negative geothermal gradients above and below the depth of the local temperature maximum may represent stagnant hot mud along the mudflow migration path, indicating a pulsative eruption of the mudflow. Gas hydrate is stable only within a few meter thick layer near the seafloor above the path because of the cold bottom water (0.4°C) covering the top of the mud volcano. Furthermore, the thermal conductivity of 0.939 W/m/K and the marine heat flow of 22.5 mW/m2 at a control site outside MV420 is estimated as the background heat flow in the slope, for which little data exists.

New evidence for preservation of contemporary marine organic carbon by iron in Arctic shelf sediments

The protection of organic carbon through association with iron minerals (FeR) is an important factor in its stabilisation, long-term storage, and burial efficiency in marine sediments. However, large uncertainties still exist concerning the sources, lability, age, and composition of the organic matter associated with FeR in natural sediments. Therefore, the timing and environmental setting of the carbon-iron bonding process remain elusive. Here we use radiocarbon (Δ14C) and stable isotopes (δ13C) of downcore bulk sedimentary organic matter, benthic foraminifera and the organic carbon fraction bound to FeR to interrogate the source and age of the organic carbon pool associated with FeR in Arctic marine sediments. In the Barents Sea, we find that the organic carbon associated with FeR is younger overall than the bulk organic matter and is probably marine derived. The comparison to other investigations of OC-FeR origins reveals that in large parts of Arctic shelf regions FeR associated organic carbon is radiocarbon enriched and has a higher δ13Corg value compared to the bulk sediment, irrespective of sediment depth/age. Our findings suggest a rapid and preferential binding of fresh and marine organic matter with FeR. Hence, labile organic matter prone to decomposition is protected and stabilised, underlining the potential of the organic carbon–iron association as an efficient carbon burial mechanism.

Blue carbon stock in sediments of mangroves and seagrass ecosystems at Southeast Sulawesi, Indonesia

AbstractCoastal ecosystems store approximately 80% of carbon in sediments, which could be emitted due to degradation and deforestation. Therefore, protecting and managing these ecosystems are crucial for climate change mitigation. This study aims to determine sediment bulk density (SBD), sediment organic carbon (SOC) concentration, and sediment carbon stocks (SCS) in mangroves and seagrasses, and to elucidate how the protection scheme impacts them. We collected four sediment cores of 100 cm each from protected and unprotected mangrove areas and three sediment cores of 80 cm each from unprotected and protected seagrass areas. The average SBD in mangroves was 0.48 g cm−3 in protected areas and 0.55 g cm−3 in unprotected areas, while the average SBD in seagrass was 0.79 g cm−3 in protected areas and 0.85 g cm−3 in unprotected areas. The average SOC in mangroves was 8.65% in protected areas and 8.63% in unprotected areas, while the average SOC in seagrass was 5.89% and 6.25% in protected and unprotected areas, respectively. The average SCS in the protected area was 381 and 363 Mg C ha−1 in the mangrove and seagrass ecosystems, respectively. On the other hand, the average SCS in the unprotected areas was 449 and 422 Mg C ha−1 in mangroves and seagrass ecosystems, respectively. Although we did not find any significant differences in SCS between protected and unprotected ecosystems regardless of ecosystem types, the conservation management of blue carbon ecosystems is still needed to avoid future carbon emissions.

Determination and quantification of sedimentary processes in salt marshes using end‐member modelling of grain‐size data

AbstractEnd‐member modelling of bulk grain‐size distributions allows the unravelling of natural and anthropogenic depositional processes in salt marshes and quantification of their respective contribution to marsh accretion. The sedimentology of two marshes is presented: (1) a sheltered back‐barrier marsh; and (2) an exposed, reinstated foreland marsh. Sedimentological data are supplemented by an age model based on lead‐210 decay and caesium‐137, as well as geochemical data. End‐member modelling of grain‐size data shows that marsh growth in back‐barrier settings is primarily controlled by the settling of fines from suspension during marsh inundation. In addition, nearby active dunes deliver aeolian sediment (up to 77% of the total sediment accretion), potentially enhancing the capability of salt marshes to adapt to sea‐level rise. Growth of exposed marshes, by contrast, primarily results from high‐energy inundation and is attributed to two sediment‐transport processes. On the seaward edge of the marsh, sedimentation is dominated by coarser‐grained traction load, whereas further inland, settling of fine‐grained suspension load prevails. In addition, a third, coarse‐grained sediment sub‐population is interpreted to derive from anthropogenic land‐reclamation measures, that is material from drainage channels relocated onto the marsh surface. This process contributed up to 34% to the total marsh accretion and terminated synchronously with the end of land reclamation measures. Data suggest that natural sediment supply to marshes alone is sufficient to outpace contemporary sea‐level rise in the study area. This underlines the resilience potential of salt marshes in times of rising sea levels. The comparison of grain‐size sub‐populations with observed climate variability implies that even managed marshes allow for the extraction of environmental signals if natural and anthropogenic sedimentary processes are determined and their relative contribution to bulk sediment composition is quantified. Data series based solely on bulk sediments, however, seem to be of limited use because it is difficult to exclude bias of natural signals by anthropogenic measures.

Habitats modulate influencing factors shaping the spatial distribution of bacterial communities along a Tibetan Plateau riverine wetland

The Tibetan Plateau riverine wetland is very sensitive to global climate change. Understanding the mechanisms that maintain the spatial patterns of bacterial communities provides insight into the dominant biogeochemical processes within the plateau riverine wetlands. Nonetheless, the spatial distribution of bacterial communities along these wetlands has rarely been explored. We investigated the spatial patterns of bacterial community within rhizosphere soil, bulk soil, and sediment samples collected along the Yarlung Tsangpo riverine wetland (YTRW), the longest plateau riverine wetland in China. Our results indicated that the diversity of bacterial communities in all three habitats increased significantly along the YTRW. The slope of the linear relationship between distance and bacterial community diversity in sediment was steeper than those for bulk and rhizosphere soils. Furthermore, bacterial communities in all three habitats showed significant distance–decay relationships. A combination of historical factors (geographical distance and climatic factors) and contemporary environmental heterogeneity (edaphic properties) controlled spatial distributions of bacterial communities in all three habitats, although climatic factors were predominant. Climatic factors affected rhizosphere bacterial communities more than those in bulk soil and sediment. Co-occurrence network analysis revealed that the potential interactions between bacterial taxa may decrease along the YTRW. This field investigation highlighted that the climatic factors strongly influenced the spatial distribution of bacterial communities along the YTRW; however, habitat differences among rhizosphere soil, bulk soil, and sediment samples affected the relative importance of climatic factors on spatial distributions of the associated bacterial communities. These findings would improve the understanding of biogeochemical processes in these typical habitats and potential alterations provoked by climate change.

Variable element enrichment sources and contributions to volcanic rocks along the Lesser Antilles Island Arc

Understanding relationships between different components involved in subduction-related recycling of sediment at arc volcanic systems has long been a focus of study. It is generally understood that volcanic arc lavas incorporate materials from the down-going slab including fluids, fluid mobile elements (FME), and fluid modified large ion lithophile elements (LILE), largely derived from subducted sediments and serpentinized ultramafic rocks. Past studies have presumed bulk sediment contributions rather than addressing localized variations in sediment composition. We examine trace element variability along the Lesser Antilles Island Arc (LAIA) and show that sediment inputs via fluid transfer to the mantle wedge are necessary to explain high B (up to 50 ppm), high B/La ratios, and higher As, Cs, Rb in the central LAIA. In contrast, the northern portion of LAIA is enriched in Ba, Sr and U/Th, with low FME. Grenada has low FME abundances, low Ba/La, but higher Sm/La. Our results suggest that modification of LAIA mantle source by fluid addition is important for the central islands but find that sediment enrichment is more likely in the northern section of the LAIA. Li/Yb ratios suggest the Grenada signature is chemically more comparable to sediment from DSDP Site 144 (Demerara Rise), while Martinique magmas in the central LAIA are affiliated to sediments from Site 543 (offshore Dominica). Unexpectedly, the northern portion of LAIA is not directly relatable to Site 543 sediments but instead trends back toward a sediment source similar in composition to Site 144 but with lower B, lower La/Sm and higher Ba. Binary mixing models suggest the northern islands are likely influenced by small amounts of sediment melt similar to average Site 144 sediments, but with Ba concentrations ranging from 1,250 to 1,500 ppm. Trace element ratios then likely result from different levels of contribution from sediment packages both spatially and temporally along the arc, with some variation evident among samples from the same island. We suggest that the process of recycling subducted sediment along the arc is variable along the length of the LAIA, wherein the trace element relationships are likely strongly influenced by the types of sediment being subducted along strike.

Sediment organic matter compositional changes in a tropical rift lake as a function of water depth and distance from shore

Little is known of modern carbon cycling in East African rift lakes. These large tropical systems with anoxic bottom waters can act as test cases for carbon cycling under future climate change scenarios as well as key sediment repositories for paleolimnology studies and fossil fuel generation. This study is the first to combine stable and radiocarbon isotopic information on bulk sediment and biomarkers (n-alcohols) in an African rift lake by investigating surface sediments collected on a nearshore to offshore transect in northern Lake Malawi. This study is also the first to report radiocarbon content in water column particulate organic, dissolved organic and inorganic carbon from an African rift lake. General surface-sediment trends with increasing water depth were an increase in %TOC, and decreases in grain size, δ13C and C/N. Carbon radioisotope analyses showed that bulk sedimentary organic matter was modern across the transect. Biomarker data indicated a primarily aquatic origin for sedimentary organic matter at offshore sites; terrestrial material dominated the n-alcohol signature closest to the shore. Protokerogen stable isotope signatures were similar, and radiocarbon was somewhat depleted relative to bulk sediment organic matter. As compared to bulk organic matter, lipids were 13C-depleted throughout the sampling transect and 14C-depleted at the most nearshore location. This indicates an aged lipid carbon source and likely significant diagenetic alteration of modern sources prior to deposition in nearshore sediments, perhaps a combination of soil erosion, in-river processing, sediment resuspension and mobilization, and preferential degradation of younger organic matter in the oxic nearshore region.

How Clay Mineral Assemblages Affect Instability on the Upper Slope of the Hikurangi Subduction Zone, New Zealand

AbstractThe International Ocean Discovery Program cored Sites U1517 (Tuaheni landslide) and U1519 (Tuaheni Basin) on the Hikurangi slope, North Island, New Zealand. We employed X‐ray diffraction to document clay mineral assemblages within the muddy sediments, with the aim of testing their potential influences on slope stability. Detrital smectite dominates the clays, with average proportions of 52 wt% at Site U1517 and 53 wt% at Site U1519. Bulk sediment from Site U1517 contains up to ∼29 wt% smectite (average = 21 wt%), high enough to reduce the angle of internal friction (on average) to ∼6°. Stratigraphic sections at both sites are homogeneous; compositional excursions are not evident along inferred slip surfaces or weak layers. Smectite decreases in the “downstream” direction of the East Cape Current, and that spatial trend correlates with lower densities of slide scars. The uniformity of compositional preconditioning, however, points to other factors as determinants of slip nucleation.

Cadmium isotope systematics in sedimentary carbonate: Extending the utility of the cadmium isotope palaeo-productivity proxy

The cadmium (Cd) isotope compositions of seven Holocene (<11.7 ka) carbonate sediments, including the bulk carbonate, as well as the > 140 μm (zooplankton), <63 μm and < 20 μm (phytoplankton) fractions, and individual species of foraminifera isolated from the bulk carbonate were obtained for sub-samples processed using the Cd-cleaning and Mg/Ca cleaning methods. The respective Cd isotope systematics of each carbonate archive were then compared with the Cd isotope signature within the ocean using modern seawater data. We show that a constant Cd isotope fractionation factor of −0.25 ± 0.03 ‰ (2SE, n = 4) describes the partitioning of Cd from seawater into the carbonate phase (Δ114CdCdCO3-seawater), allowing for accurate reconstructions of the past seawater nutrient regime. The Cd isotope signature of the < 63 µm and < 20 µm (phytoplankton) fractions can display comparable values to those recorded in the modern surface ocean, and can be used to model the magnitude of Cd uptake and utilisation in the past ocean. Individual species of planktic and benthic foraminifera, which inhabit different depths of the water column, can provide a full surface to deep water column reconstruction of the Cd nutrient regime, while infaunal benthic foraminifera can be used to interrogate the Cd isotope systematics of sediment porewaters. Bulk sediment and the > 140 µm (zooplankton) fraction also retain important information relating to the overlying water column Cd isotope systematics and can serve as essential archives for palaeo-productivity reconstructions where the separation of individual components from the bulk sediment is not possible. This new Cd isotope palaeo-productivity toolkit can considerably aid reconstruction efforts and vastly improves the ability to quantify the role of phytoplankton in modulating carbon sequestration through transitions in Earth’s climate evolution.

Metal contamination in a sediment core from Osaka Bay during the last 400 years

Osaka Bay adjacent to the Kyoto–Osaka–Kobe metropolitan area was affected by severe metal pollution during the twentieth century; yet little is known about the trace metal sources and pre-industrial human activities. We have determined the elemental concentrations and zinc stable isotope ratios (δ66Zn) in bulk sediments and the trace metal concentrations in chemical fractions of a 9-m-long sediment core from Osaka Bay. Our goals were (1) to reconstruct the historical trace metal contamination, and (2) to identify anthropogenic Zn sources and the solid phases of anthropogenic trace metals. The core provided a continuous environmental record of the last 2300 years based on radiocarbon dating of molluscan shells. Copper, Zn, and Pb showed an initial enrichment from the 1670s AD, which could be caused by human activities due to an increasing population. In agreement with previous findings, the trace metal concentrations slightly increased from the 1870s, strongly increased from the beginning of the twentieth century, and peaked around 1960 before environmental pollution control laws were enacted. Increasing trace metal concentrations in the acid-labile and reducible fractions obtained by the Community Bureau of Reference (BCR) sequential extraction procedure toward the surface indicate carbonates and Mn oxyhydroxides were the primary fractions for anthropogenic trace metals. The δ66Zn values (1) were constant until the 1940s, suggesting that the average δ66Zn of industrial sources was indistinguishable from that value of the natural background, (2) showed a slight decrease from the 1950s and remained constant until the present, and (3) fell in a binary mixing process between a lithogenic (~ + 0.27‰) and an anthropogenic endmember (~ + 0.17‰), the latter likely representing a mixture of various Zn sources such as road dust, tire wear, industrial effluents, and effluents from wastewater treatment plants. We conclude the combination of Zn stable isotopes together with chemical fractions obtained by the BCR method represents a promising approach to assess the trace metal sources and their potential mobility in sediment cores from anthropogenically affected coastal areas.

A comparative study of oil shale deposition in the Upper Cretaceous Nenjiang Formation, NE China: Evidence from petrographic and geochemical analyses

The Songliao Basin located in NE China is one of the most petroliferous basins in the world. The Upper Cretaceous Nenjiang Formation is an important source rock in the basin owing to its rich development of oil shale successions. From outcrop observation, we found that the oil shale successions can be divided into two types. The type I oil shale (TY1) is composed of multiple thin oil shale layers (∼1 m) interbedded with organic-poor shale layers and is typically developed in the first member of Nenjiang Formation (K2n1), whereas the type II oil shale (TY2) developed one thick succession of oil shale (∼10–15 m) and was mainly found in the second member of the Nenjiang Formation (K2n2). To compare the difference in forming mechanisms with two types of oil shale successions, petrographic characteristics, stable isotopic compositions, and geochemical elements were analyzed from two outcrops (i.e., Yaojia Station and Houjingou Outcrops). Based on the results, a notable higher δ13Corg values are recorded in the TY1, suggesting accelerated primary productivity. The observation of tuff layers and the relatively high P and Fe contents reveal that the frequent volcanic activities would have brought nutrient supply to the Songliao paleolake, thereby stimulating primary productivity and contributing to the deposition of multiple thin-layered oil shales. In contrast, the TY2 show decreased bulk sediment δ15N suggests intensified denitrification under anoxic conditions. Combined with elevated trace elemental ratios and increasing trend of CIA, we attributed these changes to marine transgression that occurred at the beginning of K2n2 deposition. Such conditions increased the accommodation space and favored the formation of stratified, anoxic bottom waters, which might have promoted organic matter preservation and further responsible for the deposition of thick oil shale succession in K2n2.

Effects of Glucose Addition on Dynamics of Organic Carbon Fractions and cbbL-Containing Bacteria in Wetlands

Studying the CO2-assimilation potential under the effect of glucose addition is of great significance to completely comprehend the dynamic carbon cycle in wetland ecosystems. Rhizospheric sediments (RS) and bulk sediments (BS) were selected, with the addition of glucose (G) or not, and two experimental pots (RSG and BSG) and two control pots (RS and BS) were formed. Then, within 45 h of glucose addition, the sediments were sampled at intervals of 4 h for dynamic monitoring. The bacterial communities encoded by CO2-assimilating function genes (cbbL) and the corresponding activities of key enzyme (ribulose-1,5-bisphosphate carboxylase oxygenase, RubisCO), and the light fraction (LF) and heavy fraction (HF) of organic carbon (C) and nitrogen (N) of the samples were determined. The results demonstrated that the dynamic processes of glucose deposition and degradation occurred in sediments from RSG and BSG, with the greatest depositions of 2.35 and 2.48 mg·g−1 in the 4th and 12th hour, respectively. The contents of LFOC, LFON, HFOC, and HFON decreased by 171.70%, 125.45%, 8.40%, and 68.17% in the RSG pot, and decreased by 221.55%, 102.61%, 0.07%, and 74.74% in the BSG pot, respectively, which suggested the dominant activities of C and N mineralization. The FT-MIR of LF showed different changes of typical chemical bonds between RSG and BSG during the process, which further indicated irregular and inconsistent mineralization activities. The RubisCO activities in the rhizospheric sediments (52.14 nmol (g·min)−1 on average) were substantially greater than in the bulk sediments, which indicated the high potential of carbon assimilation in rhizospheric sediments. Moreover, it showed a lower trend in BSG, BS, and RS, but an increasing trend in RSG after the glucose addition, albeit the effects were recovered in the 45th hour. The cbbL-containing bacteria were more abundant in the rhizospheric sediments than in the bulk sediments, and this effect was higher than that of the glucose addition. Proteobacteria were the dominating phylum with mean values of 93.49%, and Burkholderiales was found to be the dominant order (37.74% on average). Moreover, the changes in bacterial composition between the rhizospheric sediments and bulk sediments were more pronounced than they were during the process. Therefore, the effects of glucose degradation on RubisCO activity and cbbL-containing bacteria were transient, but the effects on organic matter fractions were straightforward, which probably further change the bacterial abundance and composition.

Grain size controls on long-chain diol distributions and proxy signals in marine sediments

Long chain alkyl diols (LCDs) are lipid biomarkers that occur ubiquitously in sediments. Their abundance and distributions are increasingly used as the basis of molecular proxies for environmental parameters such as sea surface temperature (SST) via the Long chain Diol Index (LDI), and upwelling intensity and nutrient conditions (parametrized as diol indices, DI-2, and Nutrient Diol Index, NDI, respectively). Their marine producers remain the subject of debate, but in cultures, they can be found within the outer wall (algaenan) of eustigmatophytes or in Proboscia diatoms. LCDs appear to be well preserved in sediments, potentially as a result of their association with algaenan and/or minerals, but little is known of their pre-depositional histories, in particular transport dynamics. Here, 15 surface continental margin sediments as well as one high-deposition-rate sediment core (50 cm, spanning the last ~30 years) were analyzed in order to evaluate the impact of organo-mineral associations, lateral transport, and hydrodynamic sorting on sedimentary LCD signals. The abundance and distribution of LCDs in bulk sediments and corresponding grain-size fractions was determined. The highest proportion of all LCD isomers is found in the fine fraction (2 – 10 µm), which also holds the highest proportion of organic matter in relation to the other grain-size fractions. However, LCDs are also found in the other fractions (sand, coarse silt, and clay), and their concentrations are not correlated with bulk organic carbon content, indicating different preservation or transport mechanisms. LDI-SST in the bulk sediment is comparable to the mean annual SST at all sites except those influenced by upwelling and characterized by strong seasonal SST gradients. To the contrary of other biomarker-related proxies (e.g., alkenones), lateral transport does not appear to strongly affect LDI-SST in size fractions, suggesting that the intimate relationship of LCD with the algaenan may counteract the influence of hydrodynamic mineral sorting processes on related proxy signals. The difference between the fraction-weighted LCD concentration and bulk sedimentary LCD concentration indicates potential release of LCD during laboratory fractionation, suggesting degradation of algaenan or dissolution of opal frustules.

Magnetofossils and Benthic Foraminifera Record Changes in Food Supply and Deoxygenation of the Coastal Marine Seafloor During the Paleocene‐Eocene Thermal Maximum

AbstractContinental shelf sediments along the northeast continental margin of North America contain a magnetofossil‐rich interval deposited during the Paleocene‐Eocene Thermal Maximum (PETM), an abrupt global warming event that occurred ∼56 Ma. These magnetofossils were produced by magnetotactic bacteria and unidentified, iron‐biomineralizing organisms. Previous work shows three distinct magnetofossil endmembers identified from principal component analysis of first‐order reversal curves (FORC‐PCA). The relative proportions of these magnetofossil endmembers varied over time during the PETM. Benthic foraminiferal species associations from the same sediments also vary during the PETM. Here, we compare the variability in magnetofossils and benthic foraminifera and statistically test, for the first time, whether they are significantly correlated. We find that changes in the type of magnetofossils present within the sediment correspond to changes in benthic foraminiferal assemblages, thus reflecting syndepositional conditions such as changes in oxygen, nutrients, and organic carbon concentrations, rather than diagenetic processes. We also find that magnetofossil‐producing organisms may respond to specific environmental pressures differently than benthic foraminifera, and in some cases, may better record these environmental signals. Therefore, FORC‐PCA on magnetofossil‐rich sediments can be used to understand discrete changes in ocean and/or early sediment chemistry over episodes of climate change. These environmental responses are inferred from non‐destructive, reproducible magnetic measurements, adding an independent environmental proxy to more commonly studied microfossil and geochemical proxies. Notably, these measurements only require small bulk sediment samples and do not need to contain calcium carbonate, making this approach ideal where sample material is limited and the record is affected by ocean acidification.

Invasive Spartina alterniflora accelerates soil gross nitrogen transformations to optimize its nitrogen acquisition in an estuarine and coastal wetland of China

Saltmarsh plants are important components of estuarine and coastal wetlands because they regulate ecosystem nitrogen (N) dynamics. However, complex interactions between the N uptake of saltmarsh plants and soil N transformation remain unclear. Here, we conducted a series of 15N tracing experiments with native Phragmites australis, invasive Spartina alterniflora, and bulk sediment without plants to explore the effect of plants on soil N cycling. The results showed that the NH4+ and NO3− uptake rates by the saltmarsh plants were 4.62–5.38 mg N kg⁻1 d⁻1 and 1.29–2.90 mg N kg⁻1 d⁻1, respectively, and the invasive S. alterniflora had a higher N uptake than the native P. australis. The presence of saltmarsh plants promoted N mineralization and dissimilatory NO3− reduction to NH4+, increasing the available NH4+ supply for the plants. Conversely, NH4+ immobilization and autotrophic nitrification rates were drastically reduced in the presence of the saltmarsh plants, indicating that the plants were able to outcompete soil microorganisms in NH4+ acquisition. Meanwhile, heterotrophic nitrification (organic N oxidation), which accounted for 66–82% of the total nitrification, was stimulated by the saltmarsh plants. Increased heterotrophic nitrification in the saltmarsh plants helped to provide NO3− substrates to meet the needs of the soil microorganisms and the plants. The regulatory effect of the invasive S. alterniflora on soil gross N transformation was more pronounced than that of the native P. australis due to the higher N requirements of the former. Microbial carbon sources and energy sources, relevant gene abundances and exoenzyme activities were the main factors by which the saltmarsh plants regulated gross N transformations. Overall, our results show that there are various interactions between soil microorganisms and saltmarsh plants and that S. alterniflora accelerates gross N transformations in the soil to meet its large demand for N. These findings provide valuable insights into the ecological management of invasive plants in estuarine and coastal ecosystems.