Sedimentary δ15N Research Articles

Evidence for changes in subsurface circulation in the late Eocene equatorial Pacific from radiolarian‐bound nitrogen isotope values

AbstractMicrofossil‐bound organic matter represents an important archive of surface ocean environmental information. Sedimentary nitrogen (N) isotope reconstructions of surface nitrate consumption and nitrogen source changes are made using fossil diatom (autotrophs) and planktic foraminiferal (heterotrophs)‐bound organic matter with success. However, because diatoms and planktic foraminifera are poorly preserved and sedimentary organic matter content is near zero during the late Eocene, our ability to examine nutrient dynamics across this important climate transition is limited. Here we present new data exploring the use of N isotope records from radiolarian tests. A comparison of surface ocean nitrate and core top bulk and radiolarian N isotope values (as δ15N) from the equatorial Pacific indicates that radiolarian‐N records δ15N variability with fidelity but that a significant offset exists between bulk sedimentary and diatom δ15N values and those measured from radiolarians (~7.1 ± 1.1‰). A downcore profile of radiolarian δ15N values is compared to siliceous microfossil assemblage changes across the Eocene‐Oligocene boundary. Average of radiolarian‐bound δ15N values is 0.5 ± 2.0‰, which, when corrected using the offset derived from the modern surface samples, suggests that the mean nitrogen isotopic composition of the early Cenozoic eastern Pacific was not significantly different from today. The overall trend, of decreasing δ15N values with decreasing export productivity, is consistent with either a regional decline in pelagic denitrification or a large‐scale change in nutrient sources to the eastern equatorial Pacific (EEP), both linked to the cooling climate and changing intermediate water circulation. Decreasing/low δ15N values cooccur with high radiolarian species turnover at ~35.5 and 34 Ma, suggestive of a significant ecological change in the EEP, consistent with cooling and water mass distribution changes. The preliminary results suggest that radiolarian‐bound organic nitrogen represents another promising archive and underscores the fact that the different microfossil fractions must be separated to ensure robust results.

Spatiotemporal analysis of nitrogen cycling in a mixed coniferous forest of the northern United States

Abstract. Nitrogen (N) is the limiting nutrient to primary productivity in a variety of temperate forests, and N cycling is undergoing a variety of anthropogenic changes, notably a doubling of reactive N (Nr) on a global scale. Yet, the magnitude of these changes to N cycling has been difficult to document in terrestrial ecosystems, especially in old-growth forests. To determine the trajectory of N cycling and the potential impacts of anthropogenic influences at local scales, we measured the composition of stable nitrogen isotopes (δ15N) in wood from living red pine trees (Pinus resinosa) at a single site in northern Minnesota, USA. A synchronous decline in wood δ15N values began approximately in the 1920s in 17 individual trees at different topographic positions, indicating a common driver. The decline in wood δ15N values corresponded with declines in sedimentary δ15N recorded in lacustrine sediments of the same catchment. Disturbance regime and species composition began to change at the turn of the 20th century with park establishment, providing a likely mechanism of decline in δ15N values toward present. While other mechanisms of this change are possible, we conclude that while there may be consequences of increased influxes of various forms of anthropogenic Nr into terrestrial ecosystems at the global level, these changes are not being expressed at a local level in this temperate forest ecosystem.

Late Pliocene to early Pleistocene (2.4–1.25 Ma) paleoproductivity changes in the Bering Sea: IODP expedition 323 Hole U1343E

Late Pliocene to early Pleistocene paleoproductivity changes in the Bering Sea were reconstructed using geochemical concentrations and mass accumulation rates (MARs) of CaCO3, biogenic opal, and total organic carbon (TOC), and sedimentary nitrogen isotope ratios (δ15N) at IODP Expedition 323 Hole U1343E, drilled in the northern slope area (1956m deep) of the Bering Sea. CaCO3 concentration is generally low, but prominent CaCO3 peaks occur intermittently due to subseafloor authigenic carbonate formation rather than biogenic accumulation, regardless of glacial–interglacial variations. Biogenic opal concentrations reflect orbital-scale glacial–interglacial variations. However, TOC concentration did not show clear glacial–interglacial variation, probably due to poor preservation. The sedimentary δ15N values vary synchronously with biogenic opal concentration on orbital timescales. The co-varying pattern of opal productivity and δ15N values at Hole U1343E is a result of nutrient utilization controlled by diatom productivity in the Bering slope area where Fe is not a limiting factor. Biogenic opal and TOC MARs showed a temporal shift at around 1.9Ma from a high productivity period under nutrient-enriched conditions to a low productivity period under relatively nutrient-depleted conditions. High diatom productivity with low δ15N values before 1.9Ma is associated with abundant nutrient supply by upwelling in relation to strong surface current system. This productivity decrease at about 1.9Ma was also found in the southern Bering Sea (Site U1341) and may be related to global opal reorganization.

Evaluation of redox versus thermal maturity controls on δ15N in organic rich shales: A case study of the Woodford Shale, Anadarko Basin, Oklahoma, USA

Bulk sedimentary nitrogen isotopes (δ15Nbulk) have been primarily used to identify bottom water redox states during deposition in sedimentary environments that have not undergone significant diagenetic or catagenetic alteration. Recently, the utility of sedimentary δ15Nbulk as a paleoredox proxy in hydrocarbon-bearing shale intervals was demonstrated by qualitatively correlating the δ15Nbulk profiles from shale units to their depositional redox conditions. However, the effect of thermal maturity on the δ15Nbulk signal remains unknown. We analyzed samples from three cores from the Devonian–Mississippian age Woodford Shale in the Anadarko Basin with vitrinite reflectance values ranging from the oil window to the gas window in order to investigate how depositional redox conditions and thermal maturation affect the bulk sedimentary δ15N signals. Our results indicate that the δ15Nbulk values for the Woodford do not correlate with thermal maturity. Instead, the δ15Nbulk values primarily reflect the depositional redox conditions, which are supported by trace metal concentrations and depositional features such as burrow abundance and lamination. The expected relationship between δ15Nbulk and paleoredox conditions was observed both within each core and between cores on a basin wide scale, with samples deposited under suboxic bottom water conditions having higher δ15Nbulk values than those deposited under anoxic bottom water conditions. Our data also indicate that redox bottom water column conditions in the Anadarko Basin varied spatially and temporally during the deposition of the Woodford Shale.

The influence of rapid, millennial scale climate change on nitrogen isotope dynamics of the Cariaco Basin during marine isotope stage 3

AbstractUnderstanding changes to the marine nitrogen cycle on millennial and shorter time scales can help determine the influence of rapid climate change on the fixed N pool and its sources and sinks. Rapid changes in denitrification have been observed in the eastern tropical North Pacific (ETNP) and Arabian Sea; however, millennial scale δ15N records in regions influenced by N2 fixation are sparse. We present a sedimentary δ15N record from the Cariaco Basin during marine isotope stage (MIS) 3 (~35–55 ka). The δ15N record displays a pattern of millennial scale variability that tracks the Greenland ice core Dansgaard‐Oeschger cycles, with higher values observed during interstadial periods, lower values during stadial periods, and abrupt transitions in between. Conditions during interstadials are similar to those at present in the Cariaco Basin, with the sedimentary δ15N signal reflecting a combination of local processes and an imported regional signal. If interpreted to reflect regional processes, the interstadial δ15N values (average ~5.1‰) support the argument that N2 fixation did not increase in the tropical North Atlantic during the last glacial. The lower δ15N values during stadials, when lower sea level resulted in increased physical isolation of the basin, can be explained primarily by local processes. In spite of the importance of local processes, striking similarity is observed between the Cariaco record and millennial scale δ15N records from the ETNP and Arabian Sea. The apparent synchronicity of changes observed in all three regions suggests an atmospheric teleconnection between the three sites and high‐latitude climate forcing during MIS 3.

Compound-specific δ15N and chlorin preservation in surface sediments of the Peru Margin with implications for ancient bulk δ15N records

Understanding the processes that control the preservation of paleoceanographic proxies is of clear importance. Surface sediments from the Peru Margin oxygen-minimum zone are subject to lateral and downslope transport by bottom currents that decrease organic matter (OM) quality. Indicators of bulk OM quality (pyrolysis hydrogen index, pyrolysis S1+S2 and C/N) demonstrate significant degradation between 150 and 400m water depth, within the oxygen-minimum zone. Concentrations of the three most abundant chlorins (chlorophyllone, pheophytin and pyropheophytin) decrease from 750 to 150nmol gTOC−1 from 150 to 400m water depth though the relative abundances of the chlorins in an individual sample do not change. This suggests that the three chlorins have similar reactivity over the ambient conditions. Values for δ15N of bulk sediments (δ15Nbulk) decrease by 3‰ from the inner shelf to the upper slope (1000m) but co-occurring compound-specific δ15N values (δ15Nchlorin) do not decrease downslope. The low variability of δ15Nchlorin values supports a single source for the chlorins, and demonstrates the recalcitrance of δ15Nchlorin values despite degradation. This set of observation raises questions about which type of OM fraction best records ‘primary’ signatures. We assess two possible models to guide our interpretation of these disparate datasets (1) that decreasing δ15Nbulk values are the result of degradation of a 15N-enriched fraction during downslope transport, and that δ15Nchlorin values reflect primary values; (2) that δ15Nbulk values are primary and that chlorins are derived from material transported from upslope. These data reaffirm that in active sedimentary environments such as the Eastern Tropical Pacific, transport of OM can significantly alter bulk geochemical parameters of OM integrity, but the impacts on the δ15N record of bulk sediments and chlorins are less clear, and require more study to be thoroughly understood.

Benthic biogeochemical responses to changing estuary trophic state and nutrient availability: A paired field and mesocosm experiment approach

To test how benthic biogeochemistry responds to changing nutrient availability and estuary trophic state, I conducted mesocosm and field experiments. In the mesocosm experiment, sediment cores from eutrophic and oligotrophic estuaries were incubated under high or low nutrient levels. Net production rates were greater in eutrophic vs. oligotrophic cores while respiration rates were greater in high nutrient treatments, indicating that sediment metabolism was affected by trophic history and nutrient availability. Sediment organic matter (SOM) composition was insensitive to the nutrient treatments but varied with trophic history as carbon : nitrogen ratios were lower, δ15N was higher, and algal and bacterial fatty acids (FAs) were more abundant in eutrophic sediments. In the field experiment, intact cores were reciprocally transplanted between eutrophic and oligotrophic estuaries and changes in SOM composition were followed over four months. SOM responses were more dramatic in cores transplanted to the oligotrophic vs. eutrophic estuary. In sediments moved from the eutrophic to oligotrophic estuary, carbon and nitrogen content as well as abundances of algal and bacterial FAs fell within one month. Conversely, sediment δ15N increased sharply while algal and bacterial FAs rose slightly in cores transplanted from the oligotrophic to eutrophic estuary. Different SOM responses between the experiments were likely due to direct vs. indirect effects of nutrient enrichment. Nutrient levels were manipulated in the mesocosm experiment while the estuaries differed in nutrients, macroalgal biomass, benthic animal communities, and oxygen levels. Consequently, sediment biogeochemical responses to watershed nutrient mitigation will likely follow broader changes in estuary habitat quality.

Recent changes in production in oligotrophic Uinta Mountain lakes, Utah, identified using paleolimnology

We use multiple proxies from lake sediment records of six remote alpine lakes in the Uinta Mountains, Utah, to investigate primary production and potential drivers of changes in trophic status over the last two centuries. Chlorophyll a, chlorophyll a flux, and percentage of organic matter (determined by loss on ignition) increase beginning in the mid‐20th century in five of the six alpine study lakes, indicating increasing production. These changes tend to coincide with decreasing sedimentary δ15N and increasing abundance of the nitrophilous diatom Asterionella formosa. An earlier, more subtle change in diatom community composition and δ15N indicates that human activities prior to 1950 had measurable effects on these lake ecosystems, although no corresponding increase in primary production was observed at this time. Multiple factors can lead to increased primary production at alpine sites, but the evidence presented shows that enhanced atmospheric deposition of nitrogen and phosphorus explain the recent, more prominent increase in production. Although previous research has suggested that alpine lakes are nitrogen limited, our results suggest nutrient limitation varies spatially and temporally, and is complex in these oligotrophic systems. This and other factors, such as catchment characteristics, will affect the sensitivity of a lake to atmospheric deposition–fertilizing effects. The changes observed show that remote lakes are vulnerable to long‐distance transport of nutrients, and that the risk of eutrophication could be intensified by increased nutrient inputs from expanding mining, fossil fuel combustion, and agriculture and by rapid warming predicted for the southwest.

Defining drivers of nitrogen stable isotopes (δ15N) of surface sediments in temperate lakes

The nitrogen stable isotopic signature (δ15N) of sediment is a powerful tool to understand eutrophication history, but its interpretation remains a challenge. In a large-scale comparative approach, we identified the most important drivers influencing surface sediments δ15N of 65 lakes from two regions of Canada using proxies that reflect watershed nitrogen (N) sources, internal lake microbial cycling and productivity. Across regions, we found that water column total nitrogen (TN), %N in the sediments and lake morphometric variables were the best predictors of sedimentary δ15N, explaining 66 % of its variation. Significant relationships were also found between sediment δ15N and human-derived N load ( $$ R_{{{\text{adj}} .}}^{2} $$ = 0.23, p < 0.001), the latter being a strong predictor of TN ( $$ R_{{{\text{adj}} .}}^{2} $$ = 0.68, p < 0.001). Despite a relatively strong overall relationship, variation partitioning revealed an interesting difference in the dominant variable that influenced regional δ15N. Alberta lake sedimentary δ15N signature was dominated by human derived N load. In contrast, internal processing appeared to be more important in Quebec lakes, where sediment δ15N was best explained by %N in the sediments and lake volume. Overall, our findings support the use of δ15N in paleolimnological investigations to reconstruct changing N sources to lakes but also highlight that regions may have distinctive drivers. Interpretations of sediment δ15N are likely to be strongest when multiple lines of evidence are employed and when placed in a regional context.

Compound specific amino acid δ15N in marine sediments: A new approach for studies of the marine nitrogen cycle

The nitrogen (N) isotopic composition (δ15N) of bulk sedimentary N (δ15Nbulk) is a common tool for studying past biogeochemical cycling in the paleoceanographic record. Empirical evidence suggests that natural fluctuations in the δ15N of surface nutrient N are reflected in the δ15N of exported planktonic biomass and in sedimentary δ15Nbulk. However, δ15Nbulk is an analysis of total combustible sedimentary N, and therefore also includes mixtures of N sources and/or selective removal or preservation of N-containing compounds. Compound-specific nitrogen isotope analyses of individual amino acids (δ15NAA) are novel measurements with the potential to decouple δ15N changes in nutrient N from trophic effects, two main processes that can influence δ15Nbulk records. As a proof of concept study to examine how δ15NAA can be applied in marine sedimentary systems, we compare the δ15NAA signatures of surface and sinking POM sources with shallow surface sediments from the Santa Barbara Basin, a sub-oxic depositional environmental that exhibits excellent preservation of sedimentary organic matter. Our results demonstrate that δ15NAA signatures of both planktonic biomass and sinking POM are well preserved in such surface sediments. However, we also observed an unexpected inverse correlation between δ15N value of phenylalanine (δ15NPhe; the best AA proxy for N isotopic value at the base of the food web) and calculated trophic position. We used a simple N isotope mass balance model to confirm that over long time scales, δ15NPhe values should in fact be directly dependent on shifts in ecosystem trophic position. While this result may appear incongruent with current applications of δ15NAA in food webs, it is consistent with expectations that paleoarchives will integrate N dynamics over much longer timescales. We therefore propose that for paleoceanographic applications, key δ15NAA parameters are ecosystem trophic position, which determines relative partitioning of 15N into source AA versus trophic AA pools, and the integrated δ15NAA of all common protein AA (δ15NTHAA), which serves as a proxy for the δ15N of nutrient N. Together, we suggest that these can provide a coupled picture of regime shifts in planktonic ecosystem structure, δ15N at the base of food webs, and possibly additional information about nutrient dynamics.

Separating the influences of diagenesis, productivity and anthropogenic nitrogen deposition on sedimentary δ15N variations

The stable isotopic composition of nitrogen (δ15N) in organic lake sediments is frequently used to infer changes in the source or cycling of N prior to sedimentation. However, diagenetic processes that occur after sedimentation can systematically alter the primary isotopic signal recorded in sedimentary organic matter and must be accounted for in order to detect changes in the δ15N of nitrogen inputs to the sediment surface. Here we present a null model that estimates the diagenetic effect on sedimentary lake δ15N records. The model was tested using lake sediment cores from relatively pristine alpine lakes in the Wind River and Teton Ranges of Wyoming, USA. Model-inferred increases in productivity were tested against independent productivity proxies (diatom accumulation rates and the planktonic to benthic ratio), and inferred changes in anthropogenic nitrogen contribution were validated with records of atmospheric nitrogen deposition from the nearby Fremont Glacier, Wyoming. Diagenetic overprinting significantly altered sediment δ15N profiles, and the degree of alteration was not constant through time. Of the cores analyzed, ∼30–70% of the variability can be explained by diagenesis alone, with the remainder explained by either a change in productivity or a change in the isotopic composition of the source material. Our null model of isotopic fractionation proved to be successful at separating the diagenetic overprinting from other causes of isotopic shifts, thereby providing environmental scientists with an analytical tool to partition the effects of diagenesis and environmental change on sedimentary δ15N values.

Biogeochemical impacts of wildfires over four millennia in a Rocky Mountain subalpine watershed.

Wildfires can significantly alter forest carbon (C) storage and nitrogen (N) availability, but the long-term biogeochemical legacy of wildfires is poorly understood. We obtained a lake-sediment record of fire and biogeochemistry from a subalpine forest in Colorado, USA, to examine the nature, magnitude, and duration of decadal-scale, fire-induced ecosystem change over the past c. 4250 yr. The high-resolution record contained 34 fires, including 13 high-severity events within the watershed. High-severity fires were followed by increased sedimentary N stable isotope ratios (δ15N) and bulk density, and decreased C and N concentrations--reflecting forest floor destruction, terrestrial C and N losses, and erosion. Sustained low sediment C : N c. 20-50 yr post-fire indicates reduced terrestrial organic matter subsidies to the lake. Low sedimentary δ15N c. 50-70 yr post-fire, coincident with C and N recovery, suggests diminishing terrestrial N availability during stand development. The magnitude of post-fire changes generally scaled directly with inferred fire severity. Our results support modern studies of forest successional C and N accumulation and indicate pronounced, long-lasting biogeochemical impacts of wildfires in subalpine forests. However, even repeated high-severity fires over millennia probably did not deplete C or N stocks, because centuries between high-severity fires allowed for sufficient biomass recovery.

Late Holocene elemental and isotopic carbon and nitrogen records from the East China Sea inner shelf: Implications for monsoon and upwelling

The East China Sea (ECS) is characterized by a relatively high riverine martial input that is associated with East Asian monsoon systems. In this study, we investigated a 272cm-long sediment core (THB-2) from the ECS inner shelf for the AMS 14C dating, grain size, total organic carbon (TOC), total nitrogen (TN), and stable carbon and nitrogen (δ13C and δ15N) isotopes to provide insights into previous changes in the monsoon-climate and the intensity of the coastal upwelling. A three end-member (riverine, deltaic, and marine) mixing model was applied to determine the temporal variations in sedimentary organic matter sources. The transport and burial of the Changjiang riverine organic carbon in the ECS inner shelf is strongly controlled by the combined effects of the monsoon-climate and human activities during the late Holocene. The sedimentary δ15N records of the THB-2 core are more complex and may be best explained by changes in the nutrient utilization in the coastal zone surface waters. The downcore variations of the marine organic matter-δ15N are closely related to the intensity of East Asian summer monsoon during the last 3.6ka. The stronger East Asian summer monsoon likely favored the intensive coastal upwelling and additional Changjiang discharge during the 3.6–2.7ka and 0.4–0ka periods, which triggered the greater relative nitrate utilization in the coastal surface water.

New insights into sea ice nitrogen biogeochemical dynamics from the nitrogen isotopes

AbstractWe report nitrogen (N) isotopic measurements of nitrate, total dissolved nitrogen, and particulate nitrogen from Antarctic pack ice in early and late spring. Salinity‐normalized concentrations of total fixed N are approximately twofold higher than in seawater, indicating that sea ice exchanges fixed N with seawater after its formation. The production of low‐δ15N immobile organic matter by partial nitrate assimilation and the subsequent loss of high‐δ15N nitrate during brine convection lowers the δ15N of total fixed N relative to the winter‐supplied nitrate. The effect of incomplete nitrate consumption in sea ice is thus similar to that in the summertime surface ocean, but the degree of nitrate consumption is greater in ice, leading to a higher δ15N for organic N (~3.9‰) than in the open Antarctic Zone (~0.6‰). Relative to previous findings of very high‐δ15N organic matter in sea ice (up to 41‰), this study indicates that it would be difficult for sea ice to explain the high δ15N of ice age Antarctic sediments. The partitioning of N isotopes between particulate and dissolved forms of reduced N suggests that primary production evolved from new to regenerated production from early to late spring. Even though nitrate assimilation raises the δ15N of nitrate, the δ15N of sea ice nitrate is frequently lower than that of seawater, providing direct evidence that the regeneration of reduced N in the ice includes nitrification, with mass and isotopic balances suggesting that nitrification supplies a substantial fraction (up to ~70%) of nitrate assimilated within Antarctic spring sea ice.

Reconstruction of anthropogenic eutrophication in the region off the Changjiang Estuary and central Yellow Sea: From decades to centuries

Anthropogenic activities are known to induce estuarine and coastal eutrophication. However, the eutrophication history over a longer time scale (e.g., over hundreds of years) is often missing, and this perspective is important for an objective assessment of recent-decades anthropogenic activities. To reconstruct eutrophication history in this region, two sediment cores were taken, core E4 in the region off the Changjiang Estuary in the coast of East China Sea, and core E2 in the central Yellow Sea. High sedimentation rate (3.8cm/yr) of core E4 enabled us to reconstruct a detailed anthropogenic eutrophication history for the past 70 years, while the history at least back to 1855 was further revealed via core E2. Sedimentary nitrogen isotopes (δ15N) in core E4 showed a gradually depleting trend from 5‰ (1930s) to 3.8‰ in the top, which is consistent with the increasing riverine nitrogen flux over the past few decades. A negative relationship was found between total sedimentary Chla (=preserved chlorophyll a+its degradation products) and δ15N (r2=0.68), suggesting the promotion of estuarine productivity by chemical fertilizer-N. Preserved diagnostic pigments ratio (peridinin/fucoxanthin) further suggests that after 1995, the influence of dinoflagellates has been increasing compared to diatoms. At a longer time scale (i.e., core E2), sedimentary δ15N also decreased from 5.1‰ (before 1855) to 4.4‰ (at top layer). As normalized fossil cyanobacterial pigment (zeaxanthin) showed a decreasing trend from before 1855 to the top of the core, we propose that the decreasing sedimentary δ15N after 1855 was not due to assimilation of atmospheric nitrogen, but due to excess nutrients input to the central Yellow Sea, which promoted primary production. This is further proved by preserved pheopigments, which continuously increased from 41.7nmolg OC−1 (before 1855) to 251nmolg OC−1 (at top layer) in core E2. Besides revealing the eutrophication history, big history events were also recorded, including the 1998 flood of the Changjiang River (core E4) and the shift of the Yellow River mouth in 1855 (core E2).

Variation in δ15N of a lacustrine sedimentary sequence in the Nanling Mountains and its potential implication for climatic conditions over the past ∼16 000 years

Although extensive studies have been performed on nitrogen isotopes in lake sediments, understanding the complexity of the δ15N variation related to past environmental and climatic conditions still remains unclear. Supported by multi-proxy records including litho-units, organic carbon isotopes, ration of total organic carbon and total nitrogen, organic matter content, bulk dry density, the accumulation rate of organic matter, median grain size, magnetic susceptibility, and Rb/Sr and Ca/Mg ratios obtained from a sedimentary sequence in a small lake (i.e., Dahu Swamp) in the eastern Nanling Mountains in South China, we deduce that variation in the δ15N values results primarily from the sources of organic matter, which are significantly influenced by climatic conditions over the past ∼16 000 years. A low (or high) lake level resulting from dry (or wet) conditions would lead to more (or fewer) vascular plant remains in the sediments and stronger (or weaker) denitrification associated with hypolimnetic anoxia, resulting in lower (or higher) δ15N values. In addition, dry (or wet) conditions would favor weakened (or strengthened) biogeochemical activity in the catchment, which could result in less (or more) external input of 15N-enriched OM, thus leading to decreased (or increased) δ15N values. Our results indicate that the lake sediment δ15N record in the Nanling Mountains has the potential to provide valuable insight into past East Asian monsoon climatedriven environmental changes.

Changes in the source of nutrients associated with oceanographic dynamics offshore southern Chile (41°S) over the last 25,000 years

In order to obtain a better knowledge of past oceanographic variability offshore southern Chile, this study reappraises the changes in the sources of nutrients over the last 25ka based on a detailed comparison of previously published nitrogen isotope and microfossil records (dinoflagellate cysts, coccoliths and diatoms) from ODP Site 1233 (41°S). Our findings support the main conclusions of Martinez et al. (2006) in the sense that both the Subantarctic Surface Water and the Gunther Undercurrent are potential sources for the recorded late Quaternary sedimentary δ15N signatures at Site 1233, with variable contributions of both sources during different time periods. This study indicates that Subantarctic Surface Water forms the main source for nutrients during the last glacial maximum (25–18.6calkaBP), the first part of the deglaciation (18.6–15.7calkaBP) and the Holocene (9.8calkaBP until present). An increased contribution of Equatorial Subsurface Water as a source of nutrients to the photic zone offshore southern Chile is observed between 14.4 and 9.8calkaBP, which is indicative for upwelling conditions at least after 13.2calkaBP as indicated by the microfossil data.

Using paleolimnology to track the impacts of early Arctic peoples on freshwater ecosystems from southern Baffin Island, Nunavut

Paleolimnological approaches can be used to determine the ways in which past Arctic peoples have affected the ecosystems in which they live, and simultaneously to reconstruct the climate and other aspects of the environment that may have influenced local populations. Here we analyze sediment cores from seven ponds on the south-western coast of Baffin Island, Nunavut, in order to assess the impacts of early Arctic peoples on freshwater ecosystems. Prior to the historic Inuit occupation, the study area was extensively inhabited by Thule culture Inuit (ca 1200–1600 AD) and by an earlier Arctic group, the Dorset culture Palaeo-Eskimos (ca 500 BC–1500 AD) and their predecessors from as early as 2500 BC. The study ponds were selected to cover a gradient of the intensity of human activity in their catchments. The ecological impacts of early hunting societies can be detected using paleolimnology because the butchering of marine mammals released nutrients that eutrophied nearby ponds and left distinct geochemical signals in the sediments. The degree of eutrophication in the small freshwater ponds depended on the length of the occupation, as well as the amount and type of marine mammals taken as primary prey items (eg, whales, walrus, or seals). All sediment cores were AMS 14C dated to establish their chronologies, and analyzed for diatoms and stable isotopes of nitrogen (δ15N). Both diatoms and sedimentary δ15N have been previously demonstrated to respond sensitively to nutrient enrichment from Inuit whalers. Our δ15N and diatom data record nutrient enrichment in lakes surrounded by either long-term Thule or Dorset settlements. The Dorset sites that were the locations of periodic seasonal gatherings did not register any evidence of eutrophication in the nearby ponds, reflecting the shorter, less intensive nature of these occupations. Similarly, nearby control ponds with no evidence of significant human activity in their catchments showed little-to-no changes in δ15N profiles and diatom assemblages. Due to slow rates of decomposition, nutrients from butchered marine animal bones continue to influence the freshwater sites into which they drain, as evidenced by higher than typical nutrient and production-related water chemistry variables.

The effects of wastewater effluent and river discharge on benthic heterotrophic production, organic biomass and respiration in marine coastal sediments

We examine effects of high river particulate flux and municipal wastewater effluent on heterotrophic organic carbon cycling in coastal subtidal sediments. Heterotrophic production was a predictable (r2=0.95) proportion (56%) of oxidized OC flux and strongly correlated with organic/inorganic flux. Consistent growth efficiencies (36%) occurred at all stations. Organic biomass was correlated with total, OC and buried OC fluxes, but not oxidized OC flux. Near the river, production was modest and biomass high, resulting in low P/B. Outfall deposition resulted in depleted biomass and high bacterial production, resulting in the highest P/B. These patterns explain why this region is production “saturated”. The δ15N in outfall effluent, sediments and dominant taxa provided insight into where, and which types of organisms feed directly on fresh outfall particulates, on older, refractory material buried in sediments, or utilize chemosynthetic symbiotic bacteria. Results are discussed in the context of declining bottom oxygen conditions along the coast.

Coupled response of rainfall and denitrification to solar forcing during the Holocene in Alfonso Basin.

The bulk sedimentary δ15N record of Alfonso Basin displayed changes both from a regional source possibly associated with changes in the isotopic signal of NO3– dissolved in Subsurface Subtropical Water and from a climate source governed by solar forcing (total solar irradiance) over the strength of the North American monsoon (i.e., rainfall), causing intermediate water suboxia and promoting denitrification in the basin. High values of bulk sedimentary δ15N (maximum of 14.5‰) agree with increases in land-derived organic matter (from –14.8% to 49.2%) and with a decrease in terrigenous percentage (from 84.8% to 61.6%) during the Middle and Late Holocene, suggesting that increased rainfall and freshwater input promotes increased stratification of the water column, which inhibits vertical mixing and produces a reduction in oxygen content in an already oxygen-poor zone.