Sedimentary δ15N Research Articles

Equatorial Pacific bulk sediment δ15N supports a secular increase in Southern Ocean nitrate utilization after the mid-Pleistocene Transition

In much of the equatorial Pacific, surface nitrate is consistently available due to upwelling of sub-Antarctic Mode Water-derived nitrate from the thermocline. However, in the low-latitude western boundary current (WBC) region, surface nitrate is limited due to reduced upwelling and less nitrate-replete thermocline water masses. In order to explore South Pacific WBC paleoceanography via upper ocean nitrate dynamics, we present a new bulk sediment δ15N record from within this WBC region at International Ocean Discovery Program Site U1486 (2°22′S, 144°36′E) in the Bismarck Sea north of New Guinea. This record spans from 1420 ka to recent – surpassing nearby sediment δ15N reconstructions by over a million years and allowing for direct comparison to low-latitude Pacific sediment core δ15N records of similar length. Core-top and down-core bulk sediment δ15N, TOC/TN and δ13Corg values indicate minimal or no terrestrial influence on the organic fraction at Site U1486, which is consistent with previous studies of modern processes in the region. After comparison, differences in orbital variability and secular trends throughout the Middle and Late Pleistocene suggest that nitrate dynamics along the equator and in the WBCs were relatively disparate. We observe that δ15N at Site U1486 is consistent with patterns of eastern Pacific denitrification, while we suggest increasing δ15N after the mid-Pleistocene Transition (MPT; ∼1250–700 ka) at Sites 806 (0°19′N, 110°30′W) and 849 (0°19′N, 160°E) is linked to increasing Southern Ocean nitrate utilization. Enhanced nitrate utilization is a key indicator of the strengthened biological pump that contributed to a reduction in atmospheric pCO2 during the last glacial. Therefore, a post-MPT increase in nitrate utilization may bolster the role of the Southern Ocean biological pump in driving the deeper and longer glacial periods of the 100-kyr world.

Distributions and potential sources of traditional and emerging polycyclic aromatic hydrocarbons in sediments from the lower reach of the Yangtze River, China

This study investigated the current contamination status and potential sources of traditional and emerging polycyclic aromatic hydrocarbons (t-PAHs and e-PAHs) in the sediments across a wide area of the Yangtze River, spanning nine cities. Fifty-seven sediment samples were collected in 2019, from which 15 t-PAHs and 11 e-PAHs were analyzed using GC-MSD. In addition, organic carbon (OC), total nitrogen (TN), and carbon and nitrogen stable isotope ratios (δ13C and δ15N) in sediments were measured to evaluate associations with PAHs contamination. OC, TN, and their stable isotope ratios showed a wide range of site-specific contents and values, indicating high variation in contamination and sources. Concentrations of t-PAHs and e-PAHs in sediments ranged from 0.6 to 200,000 ng g−1 dry weight (dw) and 1.1 to 20,000 ng g−1 dw, respectively. Hotspot sites located in Nanjing (PuKou), Taizhou (JingJiang), and Suzhou (ZhangJiaGang). PAHs contamination reflected land use type and human activity in the surrounding area. Positive matrix factorization (PMF) modeling showed that, on average (n = 57), vehicle emissions were the most dominant contribution (57%), followed by petroleum (28%) and fossil fuel combustion (15%). Sites with high PAHs contamination in sediments were of severe ecological risk. Contributions to the potential risks of PAHs were most significant in the order of dibenz[a,h]anthracene, benzo[b]fluoranthene, and benzo[a]pyrene. The primary origin of these compounds appeared to be fossil fuel combustion. The results of this study are expected to provide useful baseline data on the current contamination status and potential sources of traditional and emerging pollutants in the sediments of the Yangtze River, China.

Nitrogen Isotope Discrepancy Between Primary Producers and Sediments in an Anoxic and Alkaline Lake

Nitrogen isotope compositions (δ15N) in sedimentary rocks are extensively used to investigate the biogeochemical nitrogen cycle through geological times. This use relies on the observation that, in modern continental platforms and anoxic basins, surface sediments faithfully record the δ15N of primary producers, assuming that it was similar in the past. Over Earth’s history, however, surface environments experienced profound changes, including the transition of ammonium-dominated to nitrate-dominated waters and the transition from exclusively microbial ecosystems to ecosystems including multicellularity, which make modern environments significantly different compared to earlier ones, potentially invalidating the fundamental assumption that surface sediments faithfully record the δ15N of primary producers. In order to improve our understanding of the nitrogen isotopic information contained in the early Earth’s sedimentary rock record, we investigate here the nitrogen isotope systematics in a microbial, nitrate free and ammonium-rich modern system, the Dziani Dzaha Lake. In this modern system, the δ15N of the reduced dissolved inorganic nitrogen (i.e., NH4+ and NH3) in the water column is close to ∼7‰ . δ15N of suspended particulate matter (SPM) show a similar average value in surface waters (i.e., where SPM is massively composed of active primary producers), but increases up to 14‰ in the deeper part of the water column during periods when it is enriched in dissolved reduced species (i.e., CH4, H2S/HS− and NH4+/NH3). Surface sediments δ15N, with values comprised between 10 and 14 ‰, seem to preferentially record these positive isotopic signatures, rather than those of active primary producers. We propose here that the observed isotopic pattern is mainly linked to the assimilation of ammonium strongly enriched in 15N by isotope exchange with ammonia under basic conditions. Although ammonium assimilation seems here to be responsible for a significant isotopic enrichment due to the basic conditions, in neutral anoxic environments inhabited by similar microbial ecosystems, this process may also significantly impact the δ15N of primary producers towards more negative values. This would have strong implications for our interpretation of the Precambrian sedimentary record as this finding challenges one the fundamental hypotheses underlying the use of sedimentary δ15N in paleo-oceanographic reconstructions, i.e. that surface sediments faithfully record the δ15N of active primary producers in the photic zone.

Southern Ocean sourced waters modulate Eastern Equatorial Pacific denitrification during the Mid-Pleistocene transition

The Eastern tropical Pacific holds extensive oxygen minimum zone (OMZ) areas where pelagic denitrification occurs. Sedimentary nitrogen isotopes are believed to record their denitrification history, which provides indirect evidence for the past extent of the upper ocean oxygen depletion. We present a new high-resolution sedimentary δ15N record of ODP Site 1242 located in the Eastern Equatorial Pacific during the Mid-Pleistocene Transition. The sedimentary record shows increased (decreased) denitrification during interglacial (glacial) periods and a substantial 41 kyr power. Comparing the sedimentary nitrogen signal with proxies for organic carbon supply and paleoxygenation indicates a decoupling of the denitrification from local productivity and bottom water oxygenation. We claim that this feature is a response to the extratropical ventilation of the subthermocline waters. Increased glacial AAIW influence might have provided the EEP with oxygenated and nutrient-rich waters, explaining the decreased glacial denitrification rates observed in Site 1242 and other OMZs records. Qualitative inferences about the vertical oxygen structure depict an extensive deoxygenated glacial EEP. This scenario might have contributed to the storage of the carbon captured during the glacials of the MPT.

Crossing a critical threshold: Accelerated and widespread land use changes drive recent carbon and nitrogen dynamics in Vichuquén Lake (35°S) in central Chile

Global afforestation/deforestation processes (e.g., Amazon deforestation and Europe afforestation) create new anthropogenic controls on carbon cycling and nutrient supply that have not been fully assessed. Here, we use a watershed-lake dynamics approach to investigate how human-induced land cover changes have altered nutrient transference during the last 700 years in a mediterranean coastal area (Vichuquén Lake). We compare our multiproxy reconstruction with historical documentation and use satellite images to reconstruct land use/cover changes for the last 45 years. Historical landscape changes, including those during the indigenous settlements, Spanish conquest, and the Chilean Republic up to mid-20th century did not significantly alter sediment and nutrient fluxes to the lake. In contrast, the largest changes in the lake-watershed system occurred in the mid-20th century and particularly after the 1980s–90s and were characterized by a large increase in total nitrogen and organic carbon fluxes as well as negative shifts in sediment δ15N and δ13C values. This shift was coeval with the largest land cover transformation in the Vichuquén watershed, as native forests nearly disappeared while anthropogenic tree plantations expanded up to 60% of the surface area.

Restricted Oxygen‐Deficient Basins on the Northern European Epicontinental Shelf Across the Toarcian Carbon Isotope Excursion Interval

AbstractThe worldwide recognition of the Toarcian carbon isotope excursion (T‐CIE) in organic‐rich sedimentary rocks has been linked to an oceanic anoxic event (OAE) which implies the world's deep oceans were anoxic ∼183 Ma. The majority of independent redox observations used to build this argument were mainly obtained from T‐CIE organic‐rich sediments deposited on northern European epicontinental shelf. However, increasing evidence has shown that this shelf had limited connection with the open ocean, making it unsuitable for reconstructing the T‐CIE ocean redox structure. To unveil such controversy, we present integrated δ15Nbulk and δ15Nker from Dotternhausen profile, Germany, combined with literature data from other T‐CIE profiles. Both δ15Nbulk and δ15Nker values are predominantly between +0.3 and +2.5‰. These positive near‐zero δ15N values imply enhanced N2 fixation by cyanobacteria using molybdenum (Mo)‐based nitrogenase to compensate bioavailable N loss following quantitative denitrification and/or anammox in a strongly redox‐stratified marine setting. Such N isotope composition contradicts the typical sedimentary δ15N values (>3‰) induced by partial water‐column denitrification and/or anammox in modern‐ocean oxygen minimum zones. We rather propose the existence of local oxygen‐deficient basins on northern European epicontinental shelf where dissolved N underwent extensive denitrification and/or anammox causing bioavailable N deficiency. Mo‐based diazotrophy thus played a critical role in discriminating N isotope compositions among multiple hydrographically restricted T‐CIE marginal basins. Restricted oxygen‐depleted environments on the northern European epicontinental shelf unlikely represent the open‐ocean redox landscape. The existence of the global OAE thus needs comprehensive redox investigations on Tethys and/or Panthalassa deep‐sea T‐CIE successions to validate.

Ice Age‐Holocene Similarity of Foraminifera‐Bound Nitrogen Isotope Ratios in the Eastern Equatorial Pacific

AbstractBulk sediment δ15N records from the eastern tropical Pacific (ETP) extending back to the last ice age most often show low glacial δ15N, then a deglacial δ15N maximum, followed by a gradual decline to a late Holocene δ15N that is typically higher than that of the Last Glacial Maximum (LGM). The lower δ15N of the LGM has been interpreted to reflect an ice age reduction in water column denitrification. We report foraminifera shell‐bound nitrogen isotope (FB‐δ15N) measurements for the two species Neogloboquadrina dutertrei and Neogloboquadrina incompta over the last 35 ka in two sediment cores from the eastern equatorial Pacific (EEP), both of which have the typical LGM‐to‐Holocene increase in bulk sediment δ15N. FB‐δ15N contrasts with bulk sediment δ15N by not indicating a lower δ15N during the LGM. Instead, the FB‐δ15N records are dominated by a deglacial δ15N maximum, with comparable LGM and Holocene values. The lower LGM δ15N of the bulk sediment records may be an artifact, possibly related to greater exogenous N inputs and/or weaker sedimentary diagenesis during the LGM. The new data raise the possibility that the previously inferred glacial reduction in ETP water column denitrification was incorrect. A review of reconstructed ice age conditions and geochemical box model output provides mechanistic support for this possibility. However, equatorial ocean circulation and nitrate‐rich surface water overlying both core sites allow for other possible interpretations, calling for replication at non‐equatorial ETP sites.

Enhanced terrestrial organic matter burial in the marine shales of Yangtze platform during the early Carboniferous interglacial interval

The supply and preservation of terrestrial organic matter (TOM) during early Carboniferous glacial-interglacial intervals in South China were investigated with respect to their roles in paleo-environmental variation and organic matter (OM) accumulation in black shales. The marine redox proxies such as V/Cr, Corg/P ratios, and Mo–U covariation indicate a fully oxic water condition during the earliest Visean, while dominant oxic with occasional suboxic water conditions prevailed during the early to middle Visean. High sedimentary δ15N values (1.9–6.3‰) suggest the existence of enhanced water-column denitrification in the expanded oxygen minimum zones (OMZs) during the earliest Visean, whereas lower δ15N values (avg. 2.6‰) during the early to middle Visean indicate a decreasing water column denitrification in the reduced OMZs. Both the organic matter compositions and the relationship between Corg/N ratios and δ13Corg values indicate a significant contribution of terrestrial OM in the marine sediments, and the low abundance of marine OM is mainly attributed to the low paleoproductivity as evidenced by low P/Ti and Ba/Al ratios. One reason for such a high contribution of TOM is that a large amount of plant debris and wildfire-derived OM was preserved during Tournaisian glacier time and then released and buried rapidly in the marine sediments during the interglacial interval of Visean due to the temperature-driven glacial melting. The rapid transportation of large quantity of TOM from land to ocean may have resulted in remarkable drawdown of atmospheric pCO2 and subsequent cool climate, as evidenced by the decoupled relationship between δ13Corg and δ13Ccarb values. In summary, the sinking of TOM flux during the sea level lowstand and highstand may be the controlling factor for the paleo-environmental variation as well as OM accumulation in the Visean black shales.

Constraining the conditions of phosphogenesis: Stable isotope and trace element systematics of Recent Namibian phosphatic sediments

Modern phosphogenesis occurs on continental margins influenced by upwelling and high primary productivity. The formation of phosphatic sediments is coupled to global climate fluctuations, biological cycling of phosphorus and local redox conditions. Although the processes involved in phosphogenesis are well described, high-resolution data on the redox and stable isotope systematics in Recent in-situ phosphorites are scarce. In this contribution, we investigate the trace element and sulfur, nitrogen and organic carbon stable isotope composition of Recent in-situ phosphatic sediments off the coast of Namibia. Also, we examine the reliability of different widely used geochemical proxies in phosphatic sediments. Our results suggest a shift from sulfidic to suboxic conditions, coinciding with the maximum in solid calcium phosphate mineral concentration. This shift is accompanied by unidirectional changes in Mo and Re enrichments and TOC abundance. Relatively low pyrite δ34S values (ca −20‰) of phosphatic sediments indicate open system fractionation during phosphogenesis. The initiation of phosphogenesis is also accompanied by negative shifts in sedimentary δ13Corg and δ15N values. Phosphate associated sulfate (PAS) δ34S values are lower than modern seawater sulfate values, suggesting the involvement of chemolithotrophic sulfur oxidation. Our results show a shift in redox conditions from sulfidic to (sub)oxic, coupled with active sulfur cycling are prerequisites for phosphogenesis. Phosphatic sediments show substantial enrichments in U and V highlighting the complexity of using these elements, as well as V/(V + Ni) and V/Cr, as redox proxies particularly in phosphorites and phosphatic sediments.

Potential ecological implication of Cladophora oligoclora decomposition: Characteristics of nutrient migration, transformation, and response of bacterial community structure

During decay, the sediment microenvironment and water quality are severely affected by excessive proliferation of harmful algae such as filamentous green algae (FGA). The frequency of this FGA is increased through global warming and water eutrophication. In the present study, the degradation processes of a common advantage FGA Cladophora oligoclora and its effect on nitrogen and phosphorus nutrient structure and bacterial community composition at the sediment-water interface were investigated by stable isotope labelling and high-throughput sequencing. The results showed that the decomposition process of C. oligoclona was fast, stable, and difficult to degrade. The changes in sediment δ15N values reached 66.68 ‰ on day 40, which indicated that some of the nitrogen had migrated to the sediment from C. oligoclona litter. TN and NH4+-N in the overlying water rapidly increased between days 0–10, NH4+-N rose to 78.21% of TN on day 40, resulting in severe pollution of ammonia in the overlying water. The nitrogen forms and contents in the sediment are mainly derived from the increasing ammonia nitrogen release. The TP and IP in the overlying water increased to the highest concentrations of 6.68±0.64, 6.59±0.79 mg·L−1 during the decomposition process, respectively, resulting in the migration of phosphate to the sediments with increasing phosphorus content. The abundance of the main dominant bacterial communities, such as Acinetobacter (0.08%–62.48%) and Pseudomonas (0.13%–20.36%) in sediments and overlying water has changed significantly. The correlation analysis results suggested that the phosphorus was mainly related to the bacterial community in the overlying water, while the various forms of nitrogen demonstrated a high relevance with the bacterial community in the sediment. Our research results will be valuable in evaluating the potential ecological risk of FGA decomposition and provide scientific support for shallow lake management and submerged vegetation restoration.

Carbon and Nitrogen Stable Isotopes Evidence for the Environmental Impact of the Cage Fish Farm in Poyang Lake, China.

The carbon and nitrogen stable isotope compositions of the sediment organic matter, fish feed and fish feces were measured to assess the impact of the aquaculture waste from the cage fish farm in Poyang Lake. The results provide evidence of the non-negligible effect of aquaculture waste on the sediment organic matter. The δ13C and δ15N of sediments varied from - 27.62 to - 25.66‰ and 4.83 to 6.92‰, respectively. The sediment organic matter had a mixed source of waste feed, fish feces and plankton. The average contribution ratio of waste feed and fish feces was 53.1% and 24.6%, respectively. The aquaculture derived organic matter ratio was high with ranging from 57.6 to 95.6%. The aquaculture waste seemed to disperse widely enough to influence the study site about 2.5km distance from the cage. The dispersion and resuspension were the main two factors which can explain the great effect area found in this study.

A prominent isotopic fingerprint of nitrogen uptake by anaerobic methanotrophic archaea

Marine sediments represent the largest reservoir of methane on Earth. Anaerobic oxidation of methane (AOM) mediated by anaerobic methanotrophic archaea (ANME) and associated sulfate-reducing bacteria is the main process controlling methane emission from marine sediments. Recent laboratory studies have revealed that ANME or AOM consortia can mediate nitrogen (N) fixation and ammonium assimilation. These processes are known to preferentially utilize 14N rather than the 15N isotope. However, geochemical fingerprints of microbial nitrogen uptake in marine sediments are lacking, limiting our understanding of its role in modern and ancient sedimentary environments. Here we report prominent negative excursions of bulk sediment δ15N values (up to −5‰) at sulfate-methane transitions (SMTs) where ANME show peak abundances in sediments at four sites of the Haima seeps of the South China Sea. Such δ15N excursions are restricted to SMTs, suggesting that 15N depletion results from ANME-mediated nitrogen uptake. We posit that this process is ubiquitous at the SMT – a reaction front in coastal and continental margin sediments worldwide – resulting in similar isotope patterns. Negative δ15N excursions are consequently a promising proxy to track AOM in the geological record and to better constrain carbon and nitrogen cycling in ancient sedimentary environments.

Interpreting lacustrine bulk sediment δ15N values using metagenomics in a tropical hypersaline lake system

Nitrogen (N) is often a limiting nutrient in lacustrine systems, and bulk organic matter stable isotope ratios of N (δ15N) are widely used in lake sediment studies to interpret N source inputs and lake trophic status. Although records of lacustrine sedimentary δ15N can provide critical information relating to past environmental change, often productivity interpretations from δ15N and lacustrine fossil records yield conflicting interpretations. Furthermore, components of the internal N cycle have substantial isotopic fractionation factors, and likely wield an enormous influence on bulk lacustrine sedimentary δ15N values. Yet apart from cyanobacteria N-fixation, few studies link specific microbial, N-related activity to δ15N variability in lake sediment records. Here, we assess the relationship between lacustrine sedimentary δ15N and microbiome profiles analyzed from extracted sediment DNA using metagenomics. In a ~ 1600-year-long sediment record from a hypersaline lake located on Kiritimati, Republic of Kiribati (1.9° N, 157.4° W), both δ15N and the taxonomy annotations from five unique metagenomes vary with depth. Despite the relatively high abundance of Cyanobacteria, Bacteroidetes, and N-fixation genes in the uppermost sediment, we find the highest δ15N values of the sediment record there. These high values are likely due to denitrification, supported by a relatively high abundance of denitrification genes and taxa responsible for denitrification, such as those found in family Chromatiaceae within the Gamma-proteobacteria. In the deep sediment, N-related biochemical processes are likely suppressed considering the low energy, low nutrient subsurface environment. Low δ15N values observed in deeper sediments co-occur with genes for assimilatory nitrate reduction and ammonification. Thus, metagenomics provides greater clarity with respect to the specific, microbial processes that alter primary δ15N signatures in the subsurface sediment.

Integrated chemostratigraphy (δ13C-δ34S-δ15N) constrains Cretaceous lacustrine anoxic events triggered by marine sulfate input

During the Late Cretaceous period (~100.5–65 million years ago), two episodes of expansive lake anoxia occurred in the Songliao Basin (~3× the size of Lake Superior), NE China, either lasting less than one million years. These events, known as lacustrine anoxic events (LAEs), were the factors driving the development of the main source rocks for the most productive oilfields in China. The details underpinning the initiation of the LAEs, however, remain problematic. As a test of this scientific issue, we present time series of bulk organic carbon isotopes, pyrite sulfur isotopes, nitrogen isotopes and trace element concentrations from the Turonian–Coniacian Qingshankou Formation in the Songliao Basin. A notable δ34S excursion (~−10‰) and a spike in pyrite sulfur content are recorded in the basal Qingshankou Formation when the basin reached the maximum expansion. We attributed these changes to marine transgression, which would have brought high sulfate concentrations to the lake, thereby promoting microbial sulfate reduction (MSR). Subsequent sulfate limited by MSR and pyrite burial associated with isolation of the lake from the seawater drove a 20‰ positive δ34S shift. Note that the modes of the δ34S that fluctuations we describe from the LAEs have also been recognized in marine black shale during Oceanic Anoxic Event (OAE) 2, implying a common signature of anoxia. The carbon isotope data suggest that the maximal fractionation found in Qingshankou Member I was likely a consequence of the increased [CO2]aq in lake water and inputs from chemoautotrophs. Increased sedimentary δ15N during the interval of Qingshankou Member I implies intensified denitrification in the oxygen minimum zone, which lowered the water-column N/P ratios, thereby stimulating bacterial nitrogen fixation in Qingshankou Members II-III. By comparing and summarizing the sulfur, carbon and nitrogen cycles of the LAEs during deposition of the Qingshankou and Nenjiang formations, we argue that the forcing function underlying LAEs was enhanced sulfate levels from marine transgression, which might have intensified nutrient recycling and further regulated the lacustrine carbon and nitrogen cycles.

A Test of the Diatom‐Bound Paleoproxy: Tracing the Isotopic Composition of Nutrient‐Nitrogen Into Southern Ocean Particles and Sediments

AbstractSedimentary nitrogen isotope (as δ15N) records from the Southern Ocean provide critical constraints on surface nutrient consumption in the past and the role of Southern Ocean biophysical changes in setting atmospheric pCO2. We present a field assessment of how surface nitrate consumption is reflected in δ15N values of total nitrogen and diatom‐bound nitrogen pools of particles and sediments across the Southern Ocean along 170°W during late austral summer. Mixed layer nitrate δ15N values increase northwards associated with greater nitrate drawdown. Particles and sediments are expected to follow this trend. Contrary to expectations, surface ocean particle total nitrogen and diatom‐bound δ15N values decreased northward during the late summer, likely due to recycling of nitrogen and the assimilation of regenerated ammonium, as well as nitrate. The relationship between δ15N values of the total nitrogen and diatom‐bound pools remains relatively constant across this Southern Ocean transect, suggesting that the isotopic composition of these two surface ocean nitrogen pools are largely set by the δ15N value(s) of the assimilated nutrient(s). Surface sediment δ15N values do increase away from the region of maximum biogenic silica deposition, suggesting that the recycled nitrogen isotopic signal observed in late summer particles may not significantly impact the sedimentary record. However, the enrichment in δ15N values of the diatom‐bound pool is greater than what is expected from progressive utilization of the surface nitrate alone and not yet explained.

Ecological dynamics of a peri-urban lake: a multi-proxy paleolimnological study of Cultus Lake (British Columbia) over the past ~ 200 years

Peri-urban lakes offer many valued ecosystem services, but their vulnerability to climate change and anthropogenic disturbances increases with increasing human populations the effects and interactions of multiple stressors on lakes can lead to unexpected outcomes, affecting societal and ecological values, it is necessary to evaluate ecosystem trajectories and respective drivers in peri-urban lakes. Better management practices could thus be applied to preserve ecosystem services of peri-urban lakes. We conducted a multi-proxy paleolimnological study on Cultus Lake, British Columbia, a Canadian peri-urban lake experiencing cultural eutrophication, to reconstruct a comprehensive ecological trajectory of the lake over the past ~ 200 years. We also integrated historical data as well as historical archival information to identify the potential drivers of the changes. We identified ca. 1800–1900 CE as a reference period, reflected in muted variations across most paleo-indicators. Minor increases in sedimentary δ15N ca. 1880–1940 CE coincided with the onset of anthropogenic modifications to the Cultus Lake watershed. Signs of early eutrophication were evident by ca. 1940 CE, as indicated by increases in all sedimentary pigments. By ca. 1970–1990 CE, elevated concentrations of sedimentary cyanobacterial pigments and changes in diatom species assemblages highlighted the potential interactive effects of multiple stressors, including cultural eutrophication, climate warming and declines in the endangered Cultus Lake sockeye salmon population. Recent (ca. 1990–2008 CE) declines in sedimentary pigments and increases in cladoceran fluxes suggested an increase in top-down control of the lake food web. From the collection of changes observed in the past ~ 200 years in our study, it is clear that Cultus Lake and its associated ecosystem services would benefit from abatement of nutrient loadings from terrestrial and atmospheric sources. Our study emphasizes the complexity and interactivity of drivers in peri-urban lake ecosystems and the necessity of long-term perspectives to contextualize modern ecological conditions to inform lake and watershed management.

Holocene temperature and landscape history of southwest Greenland inferred from isotope and geochemical lake sediment proxies

Terrestrial temperature reconstructions from the ice-free margins of Greenland are critical for constraining the sensitivity of the Greenland ice sheet to past climate change. Numerous investigations have clarified the glacial history of southwest Greenland in recent decades; however, very few local quantitative temperature reconstructions exist as context for changes in the cryosphere. We examine sedimentary records from two lakes south of Nuuk, Greenland (informally named lakes T1 and T2), and investigate millennial scale Holocene climate change using a multi-proxy approach. Changes in X-Ray fluorescence spectroscopy (XRF), δ15N, δ13C and biogenic silica concentrations of bulk lake sediments mark the transition from a marine environment to isolated lakes as relative sea level fell in the early Holocene. δ18O values of subfossil insects (chironomids) from lakes T1 and T2 decreased by 2–3‰ from c. 8.8 ka BP (minimum limiting age for the onset of lacustrine sediment deposition in lake T2; ka BP = thousands of years before 1950) to 1 ka BP. Existing isotope-independent temperature reconstructions from the west coast of Greenland suggest that these changes in δ18O values accompanied gradual cooling of 2–4 °C, concomitant with decreasing summer insolation. There is widespread regional evidence that this cooling resulted in progressive regrowth of local glaciers and of the ice sheet. The regional consensus on the magnitude of this temperature change indicates that the local, modern modeled temperature - precipitation δ18O relationship cannot be used to quantify millennial-scale temperature changes at these sites through the Holocene. This reinforces that studies utilizing δ18O-based proxies to infer temperature change should carefully assess local isotope-temperature relationships over space and time.

Impact of seagrass establishment, industrialization and coastal infrastructure on seagrass biogeochemical sinks

The study of a Posidonia sinuosa sedimentary archive has delivered a millenary record of environmental change in Cockburn Sound (Western Australia). Ecosystem change is a major environmental problem challenging sustainable coastal development worldwide, and this study shows baseline trends and shifts in ecological processes in coastal ecosystems under environmental stress. The concentrations and fluxes of biogeochemical elements over the last 3,500 years indicate that important changes in ecosystem dynamics occurred over the last 1,000 years, in particular after ~1900's, probably related to establishment of seagrass meadows in the area and to local and regional human activities (industry and coastal development), respectively. The establishment of seagrasses ~1,000 years ago in the area of study is supported by the appearance of Posidonia fibres from ~40 cm soil depth until the core top, higher δ13C values indicating a larger contribution of seagrass-matter to the soil organic carbon pool, and increased concentration of fine sediments driven by the effect of seagrass canopy in enhancing sedimentation. The comparison of organic carbon, nutrients and metal concentrations and fluxes between pre- and post-establishment of seagrasses shows that seagrass establishment resulted in up to 9-fold increase in the soil biogeochemical sink. In ~1900's, shifts in the concentrations of metals, carbonates, organic carbon, sediment grain size, and δ13C and δ15N values of the organic matter were detected, demonstrating an alteration in seagrass ecosystem functioning following the onset of European settlement. Anthropogenic activities, and in particular the construction of a causeway in 1970's, enhanced seagrass soil organic carbon and metal accumulation rates by 36- and 39-fold, respectively, showing that human-made structures can enhance the biogeochemical sink capacity of seagrasses. Here we reconstruct the impact of human activities on seagrass ecosystem dynamics and blue carbon, which can inform local management of Cockburn Sound and seagrass conservation for climate change mitigation and adaptation.

The biogeochemical consequences of late Holocene wildfires in three subalpine lakes from northern Colorado

Wildfire activity has been increasing in forests of western North America over the past several decades. However, the biogeochemical effects of changing fire regimes are poorly understood. Here, we utilize sediment records from three subalpine lakes in northern Colorado (Hinman, Gold Creek, and Summit) to investigate the biogeochemical consequences of charcoal-inferred fire events over the past ∼2500 years. We measured element concentrations and stable isotope ratios (δ13C, δ15N) in lake sediments to track past biogeochemical processes. On average, fires were followed by increases in carbon (C), nitrogen (N), and sulfur concentrations in lake sediments, which lasted ∼20 years, while titanium and other metals found in terrestrial mineral material decreased. These changes were only statistically significant (p < 0.10) for nitrogen, titanium, and δ13C at Gold Creek Lake, and for sulfur and δ15N at Hinman Lake, suggesting either that the biogeochemical response to fire is variable through time or that the low temporal resolution of the proxy records (i.e., ∼19.2 years/sample) limited the ability to detect short-term impacts. Measurements of C/N, δ13C, and δ15N suggest that the sources of post-fire C and N differed among the study lakes. Sources of sedimentary organic matter include both erosion of soil organic matter and increased in-lake primary productivity, depending on site-specific watershed characteristics (e.g., vegetation, hydrology, elevation). These results suggest that if fire frequencies increase in the future, soil C and N stocks may not have adequate time to recover after fires, potentially jeopardizing the long-term biogeochemical resilience of these ecosystems.

The southern Gulf of Mexico: A baseline radiocarbon isoscape of surface sediments and isotopic excursions at depth.

The southern Gulf of Mexico (sGoM) is home to an extensive oil recovery and development infrastructure. In addition, the basin harbors sites of submarine hydrocarbon seepage and receives terrestrial inputs from bordering rivers. We used stable carbon, nitrogen, and radiocarbon analyses of bulk sediment organic matter to define the current baseline isoscapes of surface sediments in the sGoM and determined which factors might influence them. These baseline surface isoscapes will be useful for accessing future environmental impacts. We also examined the region for influence of hydrocarbon deposition in the sedimentary record that might be associated with hydrocarbon recovery, spillage and seepage, as was found in the northern Gulf of Mexico (nGoM) following the Deepwater Horizon (DWH) oil spill in 2010. In 1979, the sGoM experienced a major oil spill, Ixtoc 1. Surface sediment δ13C values ranged from -22.4‰ to -19.9‰, while Δ14C values ranged from -337.1‰ to -69.2‰. Sediment δ15N values ranged from 2.8‰ to 7.2‰, while the %C on a carbonate-free basis ranged in value of 0.65% to 3.89% and %N ranged in value of 0.09% to 0.49%. Spatial trends for δ13C and Δ14C were driven by water depth and distance from the coastline, while spatial trends for δ15N were driven by location (latitude and longitude). Location and distance from the coastline were significantly correlated with %C and %N. At depth in two of twenty (10%) core profiles, we found negative δ13C and Δ14C excursions from baseline values in bulk sedimentary organic material, consistent with either oil-residue deposition or terrestrial inputs, but likely the latter. We then used 210Pb dating on those two profiles to determine the time in which the excursion-containing horizons were deposited. Despite the large spill in 1979, no evidence of hydrocarbon residue remained in the sediments from this specific time period.