Bulk Sediment Research Articles

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.

Structural and chemical variability and provenance of lacustrine organic matter in the Miocene Nördlinger Ries post-impact sediments

The Nördlinger Ries in southern Germany contains a sequence of Miocene post-impact lacustrine sediments. To gain knowledge on the chemical composition and provenance of the organic matter (OM) through the up to 250 m thick sediment profiles, pyrolytic, organic geochemical and infrared spectroscopy measurements were performed on both sediments and concentrated OM. Type I kerogen prevails through the examined samples with high atomic H/C ratios (1.6 on average), hydrogen index (HI mostly >600 mg HC/g TOC), and spectral A- and C-factor values (mostly >0.8), but low oxygen index values (OI mostly <25 mg CO2/g TOC) for the concentrated OM. Exceptions occur in the uppermost Miocene sediments at the central site (Nördlingen-1973) and some marginal (SUBO-18) sediments containing hydrogen-lean type III kerogen having low H/C ratios (<1), HI (<300 mg HC/g TOC), spectral A- and C-factor values (<0.6), but high OI values (>40 mg CO2/g TOC). Mineral matrix effects induced by abundant clay minerals and zeolites significantly reduce the HI of bulk sediments as compared to concentrated OM. Organic sulfur-rich kerogen occurs in the basal units in the NR-1973 and occasionally in the SUBO-18 boreholes with high atomic Sorg/Corg ratios >0.04 and pyrolytic thiophenes/toluene ratio > 13. Abundant pyrolytic alkanes/alkenes, high spectral A-factor and CH2/CH3 values suggest prominent algal input to the OM for most of the samples. The aliphatic chain length of the kerogen decreases and the branching degree increases from the older, deeper to the younger, shallower deposits. While oxygen-bearing organic constituents are common, nitrogen-bearing moieties are rare. A general structural model is established for the type I sulfur-rich, Miocene kerogen of the Nördlinger Ries paleo-lake.

Reconstructing the lithium isotopic composition (δ7Li) of seawater from shallow marine carbonate sediments

Records of the lithium isotopic composition of seawater (δ7Lisw), preserved in the lithium isotopic composition of shallow-marine carbonate sediments (δ7Licarb), provide information about the links between silicate weathering and clay formation, the global carbon cycle, and Earth’s climate on geologic timescales. However, the record of δ7Lisw values in shallow marine carbonates is complicated by the effects of mineralogy (e.g., calcite vs aragonite) and diagenesis. Here we present measurements of bulk carbonate δ7Li values paired with a suite of stable isotope systems (δ44/40Ca, δ26Mg) and element abundance ratios (Li/(Ca + Mg), Sr/(Ca + Mg), Mg/(Ca + Mg)) in Neogene shallow-marine carbonates from sites in the Bahamas and southwest Australia. The studied sites span a range of depositional and diagenetic settings and exhibit large stratigraphic trends in δ7Li values that correlate with mineralogy, δ44/40Ca values, and/or element abundances. These trends differ from coeval planktonic foraminifera records of δ7Lisw and instead predominantly reflect local processes. We show, using a suite of geochemical analyses and a numerical model of early marine diagenesis, that the observed variability in bulk sediment δ7Li values can be quantitatively explained by the effects of mineralogy and diagenetic alteration under both fluid-buffered and sediment-buffered conditions. Using this framework, we show that it is possible to produce robust and accurate ‘snapshots’ of the δ7Li value of seawater in the geologic past from shallow-water marine carbonate sediments.

Sedimentary response to the contrasting provenance and oceanographic conditions on the continental shelf of India

The present study sets out to elucidate how magnetic minerals derived from wide array of source regions respond to shelfal sedimentary processes in a complex shelf system. Bulk sediment magnetic mineralogy comprised of detrital magnetic (magnetite, titanomagnetite, titanohematite) particles with variable concentrations and grain sizes. A prominent change in magnetic mineral contribution from mafic to felsic dominated sediment province is clearly evident through a trend of decline in magnetite content manifested by fining of magnetic crystal size. Elevated magnetite content followed by the presence of fine and coarse-grained magnetic particles, and mere variations interms of mineralogy can be attributed with the enhanced magnetic contribution from the Deccan basalt (DcB) in the shelf sediments off Narmada, Tapti, Mahi, and Sabarmati (NTMS) River system, Arabian Sea. A distinct shift in magnetite content accompanied by abrupt fluctuations in clastic grain size in the sediment core off Ganga-Brahmaputra (G-B) shelf, Bay of Bengal indicate the major change in detrital sedimentation. A major change in the magnetic mineralogy linked with enhanced supply of titanomagnetite particles marks the onset of mafic sourced material in the sediments off Pennar – Swarnamukhi (P–S) shelf, Bay of Bengal. Magnetogranulometry data revealed that magnetic crystal size is independent of physical grain size above >40 μm in NTMS and G-B, and >20 μm in P–S samples. Decoupling is largely due to polycrystalline nature, where fine-grained magnetic particles occurred as inclusions within silt and sand fractions. We demonstrate that multi-proxy approach presented in this study can be effectively used to assess the influence of the contrasting provenance and oceanographic conditions on the shelfal sediment dynamics.

Lead isotopic fingerprinting as a tracer to identify the sources of heavy metals in sediments from the Four Rivers’ inlets to Dongting Lake, China

In this study, Pb isotopic composition of bulk sediments and 1 M HCl leachates/residues of samples from the Four Rivers’ inlets to Dongting Lake (China) was determined to identify and differentiate the source of Pb inputs. Results highlighted that Xiangjiang river (XY-QS) sediments were highly enriched in Pb with less radiogenic Pb isotopes (e.g., 206Pb/207Pb ratios of 1.1830 ± 0.003), indicating the predominance of anthropogenic Pb inputs. In contrast, sediments of the Zijiang, Yuanjiang and Lishui rivers (ZD-LH) were moderately enriched in Pb and exhibited more radiogenic Pb (e.g., 206Pb/207Pb ratios of 1.1880 ± 0.008). Results from 1 M HCl extraction of XY-QS sediments illustrated that anthropogenic Pb was removed, which resulted in differentiation of 206Pb/207Pb isotope ratios between leachates (1.1740 ± 0.005) and residues (1.1846 ± 0.008), however, this difference was not observed in ZD-LH sediments (leachates: 1.1887 ± 0.009; residues: 1.1887 ± 0.008). Residual Pb isotope ratios suggested that Pb isotopic composition of lithologically inherited Pb in inlet sediments was similar, which was attributed to mixing, hydrological sorting, and averaging of different Pb sources from diverse geological settings. Pb isotopic tracing showed that Pb in XY-QS sediments resulted from the mixing of two end member sources, an anthropogenic source (less radiogenic Pb) from downstream ore smelting and upstream ore mining activities (53%) and a lithological source (37%) from granite through weathering. In contrast, Pb in ZD-LH sediments resulted from the mixing of multiple sources including Pb from upstream mining activities (20%), coal combustion (20%), and lithologically inherited Pb (60%) while surface sediments from the Yuanjiang river contained Pb from diesel oil usage (50%). Results from this study highlight the utility of a Pb isotope approach for identifying and differentiating Pb source inputs in diverse sediments from the Four River inlets to Dongting Lake (China).

Supernova versus cosmic ray origin for exotic nuclides in geomaterials: A test using 3He with 60Fe in marine sediments

We report 3He and 4He concentrations in 57 sediment samples from the southeast Indian Ocean where 60Fe excesses were previously identified in a subset of the same samples (Wallner et al., 2016). The coupled 60Fe-3He data allow further evaluation of two competing hypotheses: (1) a nearby supernova (SN) showered Earth with exotic radionuclides such as 60Fe during the last 3 million years, or (2) 60Fe in terrestrial archives was generated by reactions of galactic cosmic rays (GCRs) on micrometeorite grains that were irradiated for hundreds of millions of years in the interstellar medium, where 3He production by GCRs is larger than the solar wind 3He flux.Piston core ELT49-53 sediments show no correlation between 3He and 60Fe, and sedimentary 3He appears to be dominated by the presence of interplanetary dust particles (IDPs). Because 3He is not supplied in significant amounts by SN ejecta, the absence of a 3He-60Fe correlation provides additional, although circumstantial evidence for the supernova hypothesis. Large uncertainties in the relatively small number of sediment 60Fe measurements currently limit a firmer conclusion.The extraterrestrial 3He accumulation rate in ELT49-53 from 3.2 to 1.7 Ma was 0.88 ± 0.26 × 10−12 cm3 STP/g/kyr, similar to IDP 3He flux estimates from previous sedimentary and ice core records that span both shorter and longer time scales. 4He and 60Fe accumulation rates during this time interval were 0.11 ± 0.04 × 10−6 cm3 STP/cm2/kyr and 1.9 ± 0.5 × 104 atoms/cm2/kyr. Bulk sediment [4He] is strongly anti-correlated with sediment CaCO3 content, evidence for modulation of the terrigenous and cosmic sedimentary fractions primarily by changes in biogenic carbonate deposition. Although the dominant terrigenous source has not been uniquely identified at the Indian Ocean deposition site, it resembles eolian material from the continental interior of Australia, and shows a narrow range of 3He/4He (from 2 to 4 × 10−8, 0.015–0.030 RA) over the last ∼3 Myrs.

Experimental investigation of the effects of shrub filter strips on debris flow trapping and interception

Ecological engineering plays an increasingly significant role in mountain hazard control, but the effect of species selection and arrangement (e.g., row spacing and stem spacing) on debris flow suppression is still unclear. To further understand the interception efficiency of shrub arrangement parameters on debris flow and explore the difference with slow hydraulic erosion, sixteen sets of small-scale flume experiments with different stem and row spacings were done to study the effects of shrubs on debris flow severity, flow rate, velocity, and particle size. The results suggest that, for a dilute debris flow, sediment interception effectiveness (27.4%–60.9%) decreases gradually as stem spacing increases. Moreover, as row spacing increases, flow velocity reduction (34.4%–44.9%) and flow reduction (18.5%–47.4%) gradually decrease; and the bulk density reduction (0.5%–5.3%) and sediment interception increase initially and then decrease. In contrast, for a viscous debris flow, the flow reduction, flow velocity reduction, and sedimentation interception decrease gradually as the stem spacing increases. As row spacing increases, the flow velocity reduction, flow reduction, and sediment interception all increase initially and then decrease. A formula for the flow velocity of dilute debris flow after the filter strip was derived based on the energy conservation law and Bernoulli's equation, confirming that debris flow movement is closely related to the degree of vegetation cover. This research strengthens the current understanding of the effectiveness of vegetation in debris flow disaster prevention and control and can guide practical applications.

Root exudates release from Myriophyllum aquaticum and effects on nitrogen removal by constructed wetlands

The root exudates released from macrophytes are critical for pollutant removal and associated microorganisms, but their role in nitrogen removal remains poorly understood in constructed wetlands (CWs). The release of root exudates by Myriophyllum aquaticum was quantified using a hydroponic experiment, and then four treatments with and without environmentally relevant concentrations of root exudates were conducted to investigate the effect of root exudates on nitrogen removal in CWs. The dissolved organic carbon (DOC) release rate of M. aquaticum was 1.1–2.2 mg g−1 h−1, significantly higher than that of Typha orientalis (0.58–0.78 mg g−1 h−1) and Iris pseudacorus (0.13–0.14 mg g−1 h−1). The DOC release rate of M. aquaticum was affected by the ammonium concentration and harvesting frequency. The removal efficiencies of ammonium and total nitrogen were higher in the CWs with plants than in those without plants, confirming the crucial role of plants in plant-microbe interactions and nitrogen removal in CWs. The CWs showed significantly higher ammonium removal efficiencies under the root exudate treatments (mean 90.4% and 69.3%) than the no-root exudate treatments (87.3% and 62.0%), while total nitrogen presented no significant difference. The phylum-level taxonomic classification of microbial sequences detected in the sediments was slightly different between the root exudate and no-root exudate treatments. The results suggested that the root exudate mass naturally released by plants may be insufficient to change the carbon to nitrogen ratios of wastewater. More gene copies were observed in the rhizosphere sediments than the bulk sediments (p < 0.05), indicating that root exudates had a positive effect on microbes distribution within the rhizosphere. Therefore, high levels of root exudates as external carbon sources should be added to enhance nitrogen removal in CWs for carbon-limited wastewater treatment. The present study provided new insights into the role of macrophytes in pollutant removal and management strategy of CWs for wastewater treatment.

Radiocarbon age offset of lake sediments from central eastern China modulated by both hydroclimate and human activity

The radiocarbon (14C) dating of lake sediments is often beset by the problem of the existence of a carbon reservoir and its effect on the dated materials, which in turn influences the reliability of paleoclimatic reconstructions from lake sediments. Changes in the carbon reservoir of lake sediments are usually attributed to changes in old carbon input from the surrounding catchment caused by climatic variations or human activities. However, there is much uncertainty regarding the main driver of changes in the carbon reservoir when both climate changes and human activities coexist. We compared the 14C ages of bulk sediment samples and charcoals with the published age-model for Lake Nvshan, which is based on terrestrial plant macrofossils. The results show that the use of charcoals 14C ages for dating the sediments of Lake Nvshan is unreliable. We then obtained a time series of changes in the carbon age offset (Δt) of the bulk sediments from Lake Nvshan. The results show that prior to a significant increase in human activities, changes in Δt coincided with changes in precipitation and in the pollen concentration/total organic carbon (TOC) contents of the lake sediments; whereas after the onset of increasing human activities, changes in Δt coincided with the size of the human population of surrounding area. Prior to the increase in human activities, a higher Δt likely resulted from decreases in rainfall and phytomass which caused a reduction in contemporaneous carbon inputs. Thereafter, a higher Δt was caused by intensive human land-use and the resulting input of more old carbon. Our findings reveal a shift in the main factors influencing the Δt of bulk sediments, which was caused by the intensification of human activities in the anthropic zone. Our findings contribute to an improved understanding of the factors modulating the Δt in lake sediments.

Early Miocene Intensification of the North African Hydrological Cycle: Multi‐Proxy Evidence From the Shelf Carbonates of Malta

AbstractDuring the Miocene (23.0–5.3 Ma) North Africa experienced both humid and arid intervals, but the underlying cause of these transitions is unknown. Earth's climate was characterized by a unipolar icehouse with a dynamic Antarctic ice sheet, which may have influenced regional hydrology through atmospheric teleconnections. However, the Miocene also witnessed the restriction of the Mesopotamian Seaway, which may have had significant climatic impacts. The Maltese il‐Blata section (Central Mediterranean) comprises Late Oligocene to Early Miocene marine deposits previously used to constrain the timing of the Mesopotamian Seaway restriction using the εNd tracer. The location of this section also makes it sensitive to climatic changes in the North African region, and biogeochemical changes in the central Mediterranean. Here, we present lithological and geochemical records of the il‐Blata section. We find a marked shift in lithology and an increase in sedimentation rate coeval with the Early Miocene (∼19–20 Ma) restriction of the Mesopotamian Seaway. Concomitant changes in bulk sediment CaCO3, Sr/Ca, K/Al, Ti/Al, Zr/Al, and Si/Ti support a major humid climate transition and associated intensification of river systems over western North Africa. We propose that these changes in North African hydroclimate reflect either a tipping point effect in a gradually warming global climate, or are the result of the initial restriction of the Mesopotamian Seaway, perhaps through consequent changes in Atlantic Meridional Overturning Circulation and the West African Monsoon. We also suggest the restriction of the Mesopotamian Seaway inhibited phosphorite deposition at low latitudes.

Two glaciers and one sedimentary sink: the competing role of the Aare and the Valais glaciers in filling an overdeepened trough inferred from provenance analysis

Abstract. The extent and distribution of glaciers on the Swiss Plateau during the Last Glacial Maximum (LGM) can be determined from the geological record. However, similar reconstructions for the glaciations that preceded the LGM are far more difficult to be made due to the destruction of suitable sedimentary records through recurring glaciations or due to the inaccessibility of preserved records. Here, we explored Quaternary sediments that were deposited during the Marine Isotope Stage (MIS) 8 glaciation at least around 250 ka, and which were recovered in a drilling that was sunk into an overdeepened bedrock trough west of Bern (Switzerland). We analyzed the sediment bulk chemical composition of the deposits to investigate the supply of the material to the area by either the Aare Glacier, the Saane Glacier, or the Valais Glacier, and we complement this investigation with the results of heavy mineral analyses and geochemical information from detrital garnet. The potential confluence of the Valais and the Aare glaciers in the Bern area makes this location ideal for such an analysis. We determined the sediment bulk chemical signal of the various lithological units in the central Swiss Alps where the glaciers originated, which we used as endmembers for our provenance analysis. We then combined the results of this fingerprinting with the existing information on the sedimentary succession and its deposition history. This sedimentary suite is composed of two sequences, Sequence A (lower) and Sequence B (upper), both of which comprise a basal till that is overlain by lacustrine sediments. The till at the base of Sequence A was formed by the Aare Glacier. The overlying lacustrine deposits of an ice-contact lake were mainly supplied by the Aare Glacier. The basal till in Sequence B was also formed by the Aare Glacier. For the lacustrine deposits in Sequence B, the heavy mineral and garnet geochemical data indicate that the sediment was supplied by the Aare and the Saane glaciers. We use these findings for a paleogeographic reconstruction. During the time when Sequence A and the basal till in Sequence B were deposited, the Aare Glacier dominated the area. This strongly contrasts with the situation during the LGM, when the Aare Glacier was deflected by the Valais Glacier towards the northeast. The Valais Glacier was probably less extensive during MIS 8, but it was potentially present in the area, and it could have been essential for damming a lake in which the material supplied by the Aare and the Saane glaciers accumulated. In conclusion, combining provenance with sedimentological data, we could document how sediment was supplied to the investigated overdeepened basin during the MIS 8 glacial period and how glaciers were arranged in a way that was markedly different from the LGM.

Free-Living Amoeba and Associated Pathogenic Bacteria in Well-Chlorinated Drinking Water Storage Tanks

Free-living amoebae (FLA) and amoeba-resistant bacteria (ARB) are of increasing concern for water utilities as they cause diseases in humans. To protect consumers, disinfectants are used to prevent colonization by these pathogens in drinking water; however, FLA possess inherent defence mechanisms that can protect them from disinfection. This study investigated the presence and abundance of these amoebae and bacteria in chlorinated drinking water storage tanks in Western Australia. Viability and molecular testing methods, in the form of culturing, quantitative polymerase chain reaction, and 16S rRNA gene amplicon sequencing, were employed to detect amoebae and bacteria present in bulk water and sediment samples. FLA and ARB were detected in water storage tanks that had free chlorine residuals above 0.5 mg/L. Viable Acanthamoeba spp. and Vermamoeba sp. were isolated from the tanks along with various other FLA also detected molecularly. Nontuberculous mycobacteria, a known ARB and opportunistic pathogen, was isolated from all tanks and sample types as well as in the growth fronts of the viable amoebae. This study illustrates the presence and persistence of FLA, ARB, and associated bacterial communities in well-chlorinated water storage tanks, highlighting the need for continued surveillance and risk management.

Attenuation of beta radiation in granular matrices: implications for trapped-charge dating

Abstract. Mineral grains within sediment or rock absorb a radiation dose from the decay of radionuclides in the host matrix. For the beta dose component, the estimated dose rate must be adjusted for the attenuation of beta particles within the mineral grains. Standard calculations, originally designed for thermoluminescence dating of pottery, assume that the grain is embedded in a homogenous medium. However, most current applications of trapped-charge dating concern sand- or silt-sized dosimeters embedded in granular sediment. In such cases, the radionuclide sources are not homogeneous but are localised in discrete grains or held on grain surfaces. We show here that the mean dose rate to dosimeter grains in a granular matrix is dependent on the grain-size distributions of the source grains and of the bulk sediment, in addition to the grain size of the dosimeters. We further argue that U and Th sources are likely to be held primarily on grain surfaces, which causes the dose rate to dosimeter grains to be significantly higher than for sources distributed uniformly throughout grains. For a typical well-sorted medium sand, the beta dose rates derived from surface U and Th sources are higher by ∼ 20 % and ∼ 30 %, respectively, compared to a homogenous distribution of sources. We account for these effects using an expanded model of beta attenuation – including the effect of moisture – and validate the model against Monte Carlo radiation transport simulations within a geometry of packed spheres.

Potassium-isotope variations of marine sediments adjacent to the Izu-Bonin Trench and Nankai Trough

Characterizing the K-isotope compositions of marine sediments is a necessary step to using this novel isotope system to investigate K cycling both in the ocean system and in subduction zones. Here we report data for 70+ samples in order to characterize the K-isotope compositions of deep-sea sediments for two drill sites in the western Pacific: (1) IODP Site C0011 adjacent to the Nankai Trough and Southwest Japan arc and (2) Site 1149 adjacent to the Izu-Bonin trench and arc. Variation in δ41K at both sites is mostly subtle, reflecting a consistently strong contribution of K from sources that have δ41K similar to the upper continental crust (UCC). Yet, in detail, the mean δ41K at both sites is about 0.05–0.10 ‰ lower than a recently proposed best estimate for UCC (δ41K = −0.44 ‰). We infer that the slightly low mean δ41K values relative to UCC are likely caused by secondary processes acting on the volcanogenic component of the sediments, though chemical weathering of (non-volcanogenic) terrigenous materials may also play a role. We measured a very low δ41K value (−1.33 ± 0.04 ‰) for one discrete ash layer at Site C0011, so volcanoclastic materials may lower the δ41K values of the bulk sediments, despite only accounting for a small fraction of the K in the sediments. Correlations between δ41K, Cr/Al, and K/Al in the upper sediments of Site 1149 were used to investigate mixing between volcanic ash and terrigenous materials. This approach indicates that mixing of isotopically light volcanic ash (with δ41K about −0.75 ‰) and terrigenous materials (about −0.46 ‰) can account for the observed variation in δ41K values in these sediments. In addition, we observe that at Site C0011 K isotopes exhibit a shift (by ~0.08 ‰) at ~250 m below the seafloor from UCC-like compositions in the upper sediments to slightly lighter compositions in the lower sediments. This shift in δ41K coincides with an abrupt change in certain physical properties (porosity and cementation) of the sediments that may relate to the extent of weathering/degradation of the dispersed volcaniclastics, with more extensively weathered materials present in the lower sediments. Finally, we find evidence for variable admixture of a high δ41K silicic volcanic ash in the lowermost sediments at Site C0011, supporting the view that heterogeneous volcaniclastics can give rise to significant δ41K variations in marine sediments.

Astronomical calibration of the latest Aptian to middle Albian in the South Atlantic Ocean

Carbon cycle perturbations dramatically affected the ocean-climate system during the Aptian–Albian transition. However, the chronology of this interval is still intensively debated. Here we present a comprehensive astronomical calibration of planktonic foraminiferal and calcareous nannofossil bioevents along a complete marine sedimentary succession, comprising the latest Aptian to middle Albian interval at drilling Core SER-03, located in the Sergipe-Alagoas Basin. The SER-03 astrochronology is based on the tuning of a high-resolution gamma ray (GR) record to the long eccentricity (405 kyr) metronome, and is integrated with high-resolution paleomagnetic records. Our results span the interval between ~114.7 and 109.5 Ma, within magnetochron C34n, and our age estimates for biostratigraphic markers agree within one long eccentricity cycle with previous estimates. We also present geochemical records, such as stable carbon isotopes (δ13C), organic carbon and carbonate contents, as well as redox-sensitive elemental ratios, measured on bulk sediments at Core SER-03. Most of these proxy records depict marked changes across the Aptian–Albian boundary, likely characterizing the local expression of Oceanic Anoxic Event 1b (OAE 1b), filling the gap for low-latitude records of this event.

Evaluating the effectiveness of heavy liquid density separation in isolating K-feldspar grains using alluvial sediments from the Hajar Mountains, Oman

Heavy liquid density separation is commonly utilised in isolating specific minerals from bulk sediment for luminescence dating. K-feldspars are commonly separated from other minerals by isolating the <2.58 g cm −3 fraction which yields infrared stimulated luminescence (IRSL) signals conducive to dating. However, initial measurements of the IRSL and thermoluminescence (TL) signals from the <2.58 g cm −3 fraction of alluvial fan sediments from the Hajar Mountains, northern Oman, were atypical of K-feldspar luminescence signals reported in the literature. In this study, three different density separates (<2.58 g cm −3 , 2.58–2.54 g cm −3 and <2.565 g cm −3 ) were trialled to attempt to isolate pure K-feldspar samples for dating, to measure K-feldspar IRSL signals which are not contaminated with inputs from other feldspar, quartz and/or heavy mineral signals. The mineralogy of these separates was explored using microscopy, X-ray diffraction and mass spectrometry. We found that despite more refined heavy liquid density separation, the proportions of K-feldspar in these separates had a maximum of 40% by weight, remaining significantly contaminated by other minerals. IRSL and TL signals were atypical of K-feldspar for all density separates and it is likely that there is a contribution of signal from IR-sensitive contaminants, such as muscovite. This highlights the importance of investigating the mineralogy of samples for luminescence dating, especially in regions where little prior work has been undertaken. • K-feldspar fractions remained highly contaminated across the three different density separations applied. • Highlights the need to test the mineralogical purity of samples resulting from standard density separation techniques. • This is particularly important where the signal contributions of IRSL-sensitive contaminants may be relatively greater.

Material Source of Sediments from West Clarion–Clipperton Zone (Pacific): Evidence from Rare Earth Element Geochemistry and Clay Minerals Compositions

The geochemistry and mineralogy of sediments provide relevant information for the understanding of the origin and metallogenic mechanism of ferromanganese nodules and crusts. At present, there are still few studies on the sediment origin of the Clarion–Clipperton Zone (CCZ) of the east Pacific, particularly on the systematic origin of sediments with a longer history/length. Here, bulk sediment geochemistry and clay mineral compositions were analyzed on a 5.7 m gravity core (GC04) obtained at the CCZ, an area rich in polymetallic nodules. The results indicate that the average total content of rare earth elements (REE), including yttrium (REY), in sediments is 454.7 ppm and the REEs distribution patterns normalized by the North American Shale Composite of samples are highly consistent, with all showing negative Ce anomalies and more obvious enrichment in heavy REE (HREE) than that of light REE (LREE). Montmorillonite/illite ratio, discriminant functions and smear slide identification indicate multiple origins for the material, and are strongly influenced by contributions from marine biomass, while terrestrial materials, seamount basalts and their alteration products and authigenic source also make certain contributions. The REY characteristics of the sediments in the study area are different from those of marginal oceanic and back-arc basins, and more similar to pelagic deep-sea sediments. Based on LREE/HREE-1/δCe and LREE/HREE-Y/Ho diagrams, we conclude that samples from the study area had pelagic sedimentary properties which suffered from a strong “seawater effect”.

Bioturbation supplying young carbon into West Antarctic continental margin sediment

Accelerator Mass Spectrometry (AMS) 14C dating is a widely applied method for establishing high-precision chronologies used to underpin late Quaternary paleoclimatic studies. Feedbacks associated with Antarctic Ice Sheets (AIS) and Southern Ocean variance are thought to have played a critical role in regulating past global climate changes since the Last Glacial Maximum. However, determining the exact role of the AIS and Southern Ocean in these changes has proven difficult because of the sparse occurrence of biogenic carbonate suitable for AMS 14C dating in Antarctica. Consequently, AMS 14C dating is often measured on acid insoluble organic matter (AIOM) from bulk sediment samples, which are often biased towards ages that are older than the depositional age, due to pervasive reworking of old carbon. In this scenario, ages can still potentially provide maximum deposition ages, but such interpretations need to be assessed carefully. Here, we document AIOM and foraminifer AMS 14C dates at the same depth intervals from Antarctic continental margin cores, and we find AIOM AMS 14C dates can range between 2000 and 16,000 years younger than those of foraminifera samples. The younger AIOM 14C dates were found in bioturbated sedimentary facies, which often have extremely low sedimentation rates in Antarctica. In cores with low sedimentation rates, the down sediment contribution of organic carbon from macro-benthos can be quite significant in the sediment column with low organic carbon concentration. After considering possibilities of contamination, foraminifer reworking, and remineralization of secondary carbonate to explain this offset, we conclude that bioturbation plays an important role in the Antarctic carbon cycle as a young carbon supplier to depth into the sediment column. This has clear implications for using condensed sedimentary sections to date post-LGM records in Antarctica. These bioturbation processes also have implications for novel radiocarbon dating methods that have recently been developed to overcome sediment reworking processes and more accurately constrain timing of past AIS retreat, including ramped pyrolysis and compound specific isotopes methods.

Water depth and transparency drive the quantity and quality of organic matter in sediments of Alpine Lakes on the Tibetan Plateau

AbstractIdentifying primary environmental drivers mediating the quantity and quality of sedimentary organic matter (OM) in climate‐sensitive alpine lakes is crucial to understanding the role of alpine lakes in greenhouse gas emissions and Earth's climate system. Here, we characterized various pools of OM of 20 alpine lakes across the Tibetan Plateau, including bulk OM, water‐soluble OM and alkaline‐extracted OM from surface sediments, and dissolved OM (DOM) from surface water. The total organic carbon (TOC) content in sediments was low (&lt; 3%), and δ13C of TOC and C : N ratios indicated limited allochthonous carbon inputs. Sedimentary water‐soluble OM and alkaline‐extracted OM were both dominated by low‐molecular‐weight, low‐aromaticity compounds with low contributions of terrestrial humic substances, suggesting that sedimentary leachable OM was primarily regulated by in‐lake sources and processes. Redundancy analysis showed that water depth, water transparency, and total phosphorus concentration in water column explained ~ 50% variance of sedimentary bulk and leachable OM, substantiating the importance of autochthonous sources and primary productivity in regulating the quantity and quality of sedimentary OM. Compared with lake surface water DOM, water‐soluble OM and alkaline‐extracted OM from sediments had higher proportions of terrestrial humic‐like substances, suggesting preferential preservation of allochthonous materials in sediments. Our results are the first to demonstrate a clear link between physical attributes and sedimentary OM in Tibetan lakes. The associated relations predict that the amount of total OM and autochthonous carbon preserved in sediments would increase due to the lake enlargement under the scenarios of climate warming and precipitation enhancement, which may amplify greenhouse gas emissions from Tibetan lakes.