Sedimentary δ15N Research Articles

Examining N cycling in the northern South China Sea from N isotopic signals in nitrate and particulate phases

AbstractNitrogen sources and dynamics, one of the key issues in marine biogeochemical cycles, remain poorly constrained in marginal seas. Here we examine the nitrogen cycle in the northern South China Sea (SCS) by combining data from previous reports with a new data set of N isotopic compositions (δ15N) of nitrate, zooplankton, and sinking particles. Average δ15N in subsurface nitrate is 4.8 ± 0.3‰, similar to that of sinking particles (δ15Nsink of 4.4‰) through the euphotic zone (EZ) collected by floating traps and to documented mean (4.6‰) for long‐term mooring traps at 200 m. This along with oft‐observed shallow nitracline (<100 m) suggests that subsurface nitrate is the primary source of new N to support export production. Moreover, δ15Nsink at the bottom of the EZ resembles those of suspended particles (4.2 ± 1.0‰) and zooplankton (5.4 ± 1.0‰) inside the EZ. High similarity in δ15N among various types of particles including zooplankton in different size fractions in the EZ implies rapid N turnover in the ecosystem. In deep waters at 2000–3000 m, however, additional particulate N fluxes are found due to lateral transport, which contain 15N‐depleted particles, resulting in a downward decreasing trend of δ15Nsink. Incorporation of lighter N by bacteria and selective export of picoplankton are proposed as alternative mechanisms contributing to low δ15Nsink in the deep waters. The significant δ15Nsink change in the deep water column makes the SCS different from most other marginal seas; thus, caution should be made when using sedimentary δ15N to reconstruct paleonitrogen processes.

Relationship between δ15N values of bulk sediments and total organic carbon concentration in response to orbital-scale biogenic opal production in the Bering slope area over the last 600 kyrs

Biogenic opal production in the surface water of the Bering Sea shows clear orbital-scale variations such as interglacial highs and glacial lows. In this study, we examined the variation patterns in δ15N of bulk sediment (δ15Nbulk) reflecting nitrate utilization, and total organic carbon (TOC) concentration in terms of organic carbon (OC) production, in response to the orbital-scale biogenic opal production during the last 600 kyrs at Site U1343 (IODP Exp. 323) in the Bering slope area. At Site U1343, δ15Nbulk value and TOC concentration do not show the distinct glacial-interglacial change. Moreover, contrary to the weak correlation between TOC and biogenic opal concentrations, TOC concentration is positively correlated with δ15Nbulk value, indicating the strong control of OC production on nitrate utilization. It should be noted that glacial terminations are characterized by high δ15Nbulk value, high TOC and low biogenic opal concentration whereas the following full interglacials are characterized by low δ15Nbulk value, low TOC and high biogenic opal concentration during the last 600 kyrs. In other words, glacial terminations are characterized by complete nitrate utilization associated with high OC production under low biogenic opal production, whereas the following full interglacial periods are characterized by decreased nitrate utilization associated with low OC production under high biogenic opal production. Such different glacial-interglacial variation patterns of OC production and nitrate utilization in response to biogenic opal production can best be explained by variable Si/C (or Si/N) uptake ratio by diatoms possibly owing to Fe availability. The glacial Fe input increase at Site U1343 is thought to be ascribed to more dust input or large discharge from the glacial Yukon River. These findings imply that Fe transport and contribution may play a significant role in governing the nutrient cycles in the Bering slope region of the Northern Hemisphere.

Eutrophication decreases salt marsh resilience through proliferation of algal mats

Globally, many estuaries are affected by nutrient loading from human land uses in the surrounding watersheds. One consequence of increased nutrient levels is proliferation of opportunistic macroalgae. We sought to understand spatial and temporal dynamics of ephemeral macroalgal mats and to examine their effects on salt marsh in a eutrophic estuary in central California. A time series analysis spanning 80years revealed that algal wrack has increased exponentially in frequency on the salt marsh, and was highly correlated with nutrient concentrations in the estuary, which have increased along with fertilizer use. Analysis of sediment δ15N showed a dramatic increase in nutrient loads attributable to agricultural fertilizer over the past 50years. We monitored 15 salt marsh plots along the bank edge and detected a negative relationship between algal wrack cover and salt marsh cover, flowering, and canopy height. Moreover, algal wrack led to retreat of vegetation from the bank edge, and increased bank erosion. We also experimentally added algal wrack to salt marsh edge plots. Algal addition decreased salt marsh cover, flowering, and canopy height, and increased retreat rate. By integrating time series analyses, isotope data, algal and marsh monitoring and manipulative experiments, we have identified robust linkages between increased anthropogenic nutrient loading, increased algal wrack cover, reduction in marsh resilience and conversion of marsh habitat to mudflat through bank erosion. Decreasing nutrient inputs to eutrophic estuaries is thus essential for conservation and restoration of salt marshes and enhancing their resilience in the face of sea level rise.

Distribution and sources of organic carbon, nitrogen and their isotopes in surface sediments from the largest mariculture zone of the eastern Guangdong coast, South China

China is the world's largest mariculture producer. However, there is limited detailed information on organic matter distribution generated by mariculture in China. This study assessed the total organic carbon (TOC), total nitrogen (TN), and isotopic composition (δ13C and δ15N) in surface sediments from Zhelin Bay in South China. TOC, TN, δ13C and δ15N ranged from 0.46–1.23%, 0.08–0.20%, −22.07 to −21.13‰ and 5.28–7.14‰, respectively, and followed similar spatial patterns. The C/N ratio coupled with isotopic composition (δ13C and δ15N), indicates that the TOC originated from both terrestrial and marine sources. In general, surface sediments of Zhelin Bay were dominated by marine derived organic carbon; this form of carbon was separately calculated based on δ13C-based two end-member mixing and C/N ratio models.

Widespread Anthropogenic Nitrogen in Northwestern Pacific Ocean Sediment.

Sediment samples from the East China and Yellow seas collected adjacent to continental China were found to have lower δ15N values (expressed as δ15N = [15N:14Nsample/15N:14Nair - 1] × 1000‰; the sediment 15N:14N ratio relative to the air nitrogen 15N:14N ratio). In contrast, the Arctic sediments from the Chukchi Sea, the sampling region furthest from China, showed higher δ15N values (2-3‰ higher than those representing the East China and the Yellow sea sediments). Across the sites sampled, the levels of sediment δ15N increased with increasing distance from China, which is broadly consistent with the decreasing influence of anthropogenic nitrogen (NANTH) resulting from fossil fuel combustion and fertilizer use. We concluded that, of several processes, the input of NANTH appears to be emerging as a new driver of change in the sediment δ15N value in marginal seas adjacent to China. The present results indicate that the effect of NANTH has extended beyond the ocean water column into the deep sedimentary environment, presumably via biological assimilation of NANTH followed by deposition. Further, the findings indicate that NANTH is taking over from the conventional paradigm of nitrate flux from nitrate-rich deep water as the primary driver of biological export production in this region of the Pacific Ocean.

A Holocene record of ocean productivity and upwelling from the northern California continental slope

The Holocene upwelling history of the northern California continental slope is examined using the high-resolution record of TN062-O550 (40.9°N, 124.6°W, 550 m water depth). This 7-m-long marine sediment core spans the last ∼7500 years, and we use it to test the hypothesis that marine productivity in the California Current System (CCS) driven by coastal upwelling has co-varied with Holocene millennial-scale warm intervals. A combination of biogenic sediment concentrations (opal, total organic C, and total N), stable isotopes (organic matter δ13C and bulk sedimentary δ15N), and key microfossil indicators of upwelling were used to test this hypothesis. The record of biogenic accumulation in TN062-O550 shows considerable Holocene variability despite being located within 50 km of the mouth of the Eel River, which is one of the largest sources of terrigenous sediment to the Northeast Pacific Ocean margin. A key time interval beginning at ∼2900 calibrated years before present (cal yr BP) indicates the onset of modern upwelling in the CCS, and this period also corresponds to the most intense period of upwelling in the last 7500 years. When these results are placed into a regional CCS context during the Holocene, it was found that the timing of upwelling intensification at TN062-O550 corresponds closely to that seen at nearby ODP Site 1019, as well as in the Santa Barbara Basin of southern California. Other CCS records with less refined age control show similar results, which suggest late Holocene upwelling intensification may be synchronous throughout the CCS. Based on the strong correspondence between the alkenone sea surface temperature record at ODP Site 1019 and the onset of late Holocene upwelling in northern California, we suggest that CCS warming may be conducive to upwelling intensification, though future changes are unclear as the mechanisms forcing SST variability may differ.

Alder, Nitrogen, and Lake Ecology: Terrestrial-Aquatic Linkages in the Postglacial History of Lone Spruce Pond, Southwestern Alaska.

Diatoms, combined with a multiproxy study of lake sediments (organic matter, N, δ15N, δ13C, biogenic silica, grain size, Cladocera and chironomids, Alnus pollen) from Lone Spruce Pond, Alaska detail the late-glacial to Holocene history of the lake and its response to regional climate and landscape change over the last 14.5 cal ka BP. We show that the immigration of alder (Alnus viridis) in the early Holocene marks the rise of available reactive nitrogen (Nr) in the lake as well as the establishment of a primarily planktonic diatom community. The later establishment of diatom Discostella stelligera is coupled to a rise of sedimentary δ15N, indicating diminished competition for this nutrient. This terrestrial-aquatic linkage demonstrates how profoundly vegetation may affect soil geochemistry, lake development, and lake ecology over millennial timescales. Furthermore, the response of the diatom community to strengthened stratification and N levels in the past confirms the sensitivity of planktonic diatom communities to changing thermal and nutrient regimes. These past ecosystem dynamics serve as an analogue for the nature of threshold-type ecological responses to current climate change and atmospheric nitrogen (Nr) deposition, but also for the larger changes we should anticipate under future climate, pollution, and vegetation succession scenarios in high-latitude and high-elevation regions.

Stratification-induced variations in nutrient utilization in the Polar North Atlantic during past interglacials

Vertical water mass structure in the Polar North Atlantic Ocean plays a critical role in planetary climate by influencing the formation rate of North Atlantic deepwater, which in turn affects surface heat transfer in the northern hemisphere, ventilation of the deep sea, and ocean circulation on a global scale. However, the response of upper stratification in the Nordic seas to near-future hydrologic forcing, as surface water warms and freshens due to global temperature rise and Greenland ice demise, remains poorly known. While past major interglacials are viewed as potential analogues of the present, recent findings suggest that very different surface ocean conditions prevailed in the Polar North Atlantic during Marine Isotope Stage (MIS) 5e and 11 compared to the Holocene. It is thus crucial to identify the causes of those differences in order to understand their role in climatic and oceanographic variability. To resolve this, we pair here bulk sediment δ15N isotopic signatures with planktonic foraminiferal assemblages and their isotopic composition across major past interglacials. The comparison defines for the first time stratification-induced variations in nitrate utilization up to 25% between and within all of these warm periods that highlight changes in the thickness of the mixed-layer throughout the previous interglacials. That thickness directly controls the depth-level of Atlantic water inflow. The major changes of nitrate utilization recorded here thus suggest that a thicker mixed-layer prevailed during past interglacials, probably related to longer freshwater input associated with the preceding glacial termination. This would have caused the Atlantic water to flow at greater depth during MIS 5e and 11. These results call for caution when using older interglacials as modern or near-future climate analogues and contribute to the improvement of our general comprehension of the impact of freshwater input near a globally important deep-water formation site like the Nordic Seas. This is crucial when assessing the negative impacts on the Greenland Ice Sheet of climate change and global warming.

Seasonal variations and sources of sedimentary organic carbon in Tokyo Bay

Total organic carbon (TOC), total nitrogen (TN) contents, their stable C and N isotope ratio (δ13C and δ15N), and chlorophyll a ([Chl a]sed) of surface sediments were investigated monthly to identify the seasonal variations and sources of organic matter in Tokyo Bay. The sedimentary TOC (TOCsed) and TN (TNsed) contents, and the sedimentary δ13C and δ15N (δ13Csed and δ15Nsed) values were higher in summer than other seasons. The seasonal variations were controlled by high primary production in the water column and hypoxic water in the bottom water during summer. The fraction of terrestrial and marine derived organic matter was estimated by Bayesian mixing model using stable isotope data and TOC/TN ratio. Surface sediments in Tokyo Bay are dominated by marine derived organic matter, which accounts for about 69±5% of TOCsed.

Effects of current regimes and oxygenation on particulate matter preservation on the Namibian shelf: Insights from amino acid biogeochemistry

Many reconstructions of past biogeochemical states rely on proxies such as δ15N that in turn are affected by the preservation state of organic matter. N-turnover processes in the upwelling system on the Namibian shelf are difficult to reconstruct because lateral particle advection and differential degradation under oxic to anoxic conditions complicate particle tracing in suspended matter and surface sediments as well as the interpretation of sedimentary δ15N records. We analysed the amino acid composition of suspended matter, sinking particles, phytoplankton biomass and surface sediments in order to investigate the degradation pathway of organic matter from the sea surface to the surface sediment. A principal component analysis carried out with the amino acid data set results in a triangular-shaped model. Two branches evolve from phytoplankton biomass on the apex: The suspended matter branch is characterized by relatively enhanced contents of Glu, Ser and Leu, whereas the non-protein amino acids and Lys mark the sinking particle and sediment branch. The results suggest that both pools have taken different diagenetic pathways and that there is only limited exchange between them. The preservation state of organic matter in surface sediments within the oxygen minimum zone is generally better than on the oxic outer shelf, where organic matter preservation depends on water depth.A vertical increase in δ15N ratios from phytoplankton biomass to the surface sediment by ~2.5‰ is attributed to early diagenetic degradation in the water column and at the water/sediment interface. The δ15N ratios of sediments are mainly controlled by the N-isotope signature of nitrate on the outer shelf and upper continental slope. Nutrient-enriched deep water ascends at the shelf breaks due to the influence of internal waves and tides or curl-driven shelf break upwelling. The δ15N of upwelling nitrate at the shelf breaks is ~3‰ lower than that of nitrate from the inner shelf. This difference in nitrate sources is expressed in a band of decreasing δ15N ratios of surface sediments at the shelf breaks. The hydrodynamic upwelling regime at the shelf breaks flushes significant amounts of suspended matter from the mesopelagic ocean across the shelf break as indicated by amino acid monomer patterns of suspended matter.

Palaeoecological evidence for sustained change in a shallow Murray River (Australia) floodplain lake: regime shift or press response?

Paleolimnological techniques can reveal long-term perturbations and associated stable state transitions of lake ecosystems. However, such transitions are difficult to predict since changes to lake ecosystems can be abrupt or gradual. This study examined whether there were past transitions in the ecological regime of Kings Billabong, a shallow River Murray wetland in southeast Australia. A 94-cm-long core, covering c. 90 years of age, was analysed at 1 cm resolution for subfossil cladocerans, diatoms and other proxies. Prior to river regulation (c. 1930), the littoral to planktonic ratios of cladocerans and diatoms, and bulk sediment δ13C values were high, while the period from c. 1930 to c. 1970 experienced considerable changes to the wetland ecosystem. The abrupt nature of changes of planktonic cladocerans and diatoms, particularly after the onset of river regulation (1930s), was triggered by inundation, high rates of sedimentation and shifts in bulk sediment δ15N values. However, the transition of a once littoral-dominated community, to one favouring an increasingly turbid, plankton-dominated trophic condition following river regulation was relatively slow and lasted for decades. The progression to a new regime was likely delayed by the partial recovery of submerged plant communities and related internal dynamics.

Composition of stable carbon and nitrogen isotopes in five wetland plants and sediments from the Pearl River estuary, South China

ABSTRACTThis study sampled five plant species and adjoining sediments from the Qi’ao (Zhuhai) and Nansha (Guangzhou) coastal wetlands located in the Pearl River estuary, South China. The compositions of stable carbon and nitrogen isotopes as well as the content of carbon and nitrogen in the samples were analysed. Differences in carbon/nitrogen (C/N) ratio, and habitat feature were compared between exotic plants (Spartina alterniflora, Sonneratia apetala, and Laguncularia racemosa) and native mangroves (Aegiceras corniculatum and Acrostichum aureum). The results showed that for Qi’ao, which is nearer the sea, the conductivity of the sediments at this location was approximately two times higher than that at Nansha (more inland). The composition of both δ13C and δ15N in sediments was also higher at Qi’ao (−26.52‰ to −23.83‰ and 6.25‰ to 11.53‰, respectively) as compared to Nansha (−29.30‰ to −27.43‰ and 3.34‰ to 4.73‰, respectively). Overall, the exotic plants S. alterniflora and S. apetala at Qi’ao and S. apetala and L. racemosa at Nansha had an average δ15N composition significantly higher than that of the native plants A. corniculatum and A. aureum. This suggests that the three exotic plants employ a different mechanism of δ15N storage and utilisation as compared to the two native mangrove species, which may result in changes in ecological and biogeochemical processes in these coast wetlands.

Extreme 15N Depletion in Seagrasses

Seagrass beds form an important part of the coastal ecosystem in many parts of the world but are very sensitive to anthropogenic nutrient increases. In the last decades, stable isotopes have been used as tracers of anthropogenic nutrient sources and to distinguish these impacts from natural environmental change, as well as in the identification of food sources in isotopic food web reconstruction. Thus, it is important to establish the extent of natural variations on the stable isotope composition of seagrass, validating their ability to act as both tracers of nutrients and food sources. Around the world, depending on the seagrass species and ecosystem, values of seagrass N normally vary from 0 to 8 ‰ δ15N. In this study, highly unusual seagrass N isotope values were observed on the east coast of Qatar, with significant spatial variation over a scale of a few metres, and with δ15N values ranging from +2.95 to −12.39 ‰ within a single bay during March 2012. This pattern of variation was consistent over a period of a year although there was a seasonal effect on the seagrass δ15N values. Seagrass, water column and sediment nutrient profiles were not correlated with seagrass δ15N values and neither were longer-term indicators of nutrient limitation such as seagrass biomass and height. Sediment δ15N values were correlated with Halodule uninervis δ15N values and this, together with the small spatial scale of variation, suggest that localised sediment processes may be responsible for the extreme isotopic values. Consistent differences in sediment to plant 15N discrimination between seagrass species also suggest that species-specific nutrient uptake mechanisms contribute to the observed δ15N values. This study reports some of the most extreme, negative δ15N values ever noted for seagrass (as low as −12.4 ‰) and some of the most highly spatially variable (values varied over 15.4 ‰ in a relatively small area of only 655 ha). These results are widely relevant, as they demonstrate the need for adequate spatial and temporal sampling when working with N stable isotopes to identify food sources in food web studies or as tracers of anthropogenic nutrients.

Different responses of sedimentary δ15N to climatic changes and anthropogenic impacts in lakes across the Eastern margin of the Tibetan Plateau

Knowledge of historical variations in sedimentary δ15N values and their relation to modern ones in lakes is critical in understanding N cycling in inland aquatic systems. Here we present late Holocene sedimentary δ15N variations in four lakes across the Eastern margin of the Tibetan Plateau (ETP), namely Lake Qinghai and Lake Chaonaqiu over north ETP (N-ETP) area, and Lake Yihai and Lake Lugu over south ETP (S-ETP) area. The results show that: (1) during historical periods when anthropogenic forcing was weak, the centennial/multi-decadal changes in δ15N values were dominated by climatic changes. Higher precipitation brought greater nutrients supply and led to higher Lacustrine Primary Productivity (LPP) and higher δ15N values in lake sediments, and vice versa. (2) During the recent/modern epoch with enhanced anthropogenic impacts, sedimentary δ15N values in Lake Qinghai and Lake Chaonaqiu (mid-high latitude lakes with low LPP) showed decreasing trends, possibly related to the increased global atmospheric reactive nitrogen (Nr) deposition. On the contrary, sedimentary δ15N values in Lake Lugu and Lake Yihai (subtropical lakes with high LPP) showed increasing trends, most likely because more dissolved inorganic nitrogen in lake water was used during LPP processes. This study highlights that LPP plays an important role in modifying sedimentary δ15N values both for historical and modern times, whereas atmospheric Nr deposition seems to influence the modern/recent sedimentary δ15N trends in lakes with low LPP.

Nitrogen deposition to lakes in national parks of the western Great Lakes region: Isotopic signatures, watershed retention, and algal shifts

AbstractAtmospheric deposition is a primary source of reactive nitrogen (Nr) to undisturbed watersheds of the Great Lakes region of the U.S., raising concerns over whether enhanced delivery over recent decades has affected lake ecosystems. The National Atmospheric Deposition Program (NADP) has been measuring Nr deposition in this region for over 35 years. Here we explore the relationships among NADP‐measured Nr deposition, nitrogen stable isotopes (δ15N) in lake sediments, and the response of algal communities in 28 lakes situated in national parks of the western Great Lakes region of the U.S. We find that 36% of the lakes preserve a sediment δ15N record that is statistically correlated with some form of Nr deposition (total dissolved inorganic N, nitrate, or ammonium). Furthermore, measured long‐term (since 1982) nitrogen biogeochemistry and inferred critical nitrogen loads suggest that watershed nitrogen retention and climate strongly affect whether sediment δ15N is related to Nr deposition in lake sediment records. Measurements of algal change over the last ~ 150 years suggest that Nr deposition, in‐lake nutrient cycling, and watershed inputs are important factors affecting diatom community composition, in addition to direct climatic effects on lake physical limnology. The findings suggest that bulk sediment δ15N does reflect Nr deposition in some instances. In addition, this study highlights the interactive effects of Nr deposition and climate variability.

Enhanced subarctic Pacific stratification and nutrient utilization during glacials over the last 1.2 Myr

AbstractThe relationship between climate, biological productivity, and nutrient flux is of considerable interest in the subarctic Pacific, which represents an important high‐nitrate, low‐chlorophyll region. While previous studies suggest that changes in iron supply and/or physical ocean stratification could hypothetically explain orbital‐scale fluctuations in subarctic Pacific nutrient utilization and productivity, previous records of nutrient utilization are too short to evaluate these relationships over many glacial‐interglacial cycles. We present new, high‐resolution records of sedimentary δ15N, which offer the first opportunity to evaluate systematic, orbital‐scale variations in subarctic Pacific nitrate utilization from 1.2 Ma. Nitrate utilization was enhanced during all glacials, varied with orbital‐scale periodicity since the mid‐Pleistocene transition, was strongly correlated with enhanced aeolian dust and low atmospheric CO2, but was not correlated with productivity. These results suggest that glacial stratification, rather than iron fertilization, systematically exerted an important regional control on nutrient utilization and air‐sea carbon flux.

A 400‐year isotopic record of seabird response to eastern tropical Pacific productivity

Seabirds play an important role in coastal environments, serving as key indicators of marine ecosystem variability as well as biovectors that influence terrestrial productivity and carbon storage. Long‐term estimates of seabird populations remain rare, but lakes that support large seabird populations in their watersheds can archive a history of seabird activity in their sediment records. Here we present a seabird guano‐influenced sediment record from Genovesa Crater Lake, Galápagos Islands, home to the world's largest reported colony of red‐footed boobies (Sula sula) and smaller populations of other species. Influx of seabird guano into Genovesa Crater Lake produces high sedimentary δ15N values, and temporal variability in sediment δ15N primarily reflects changes in guano influx through time. Two abrupt increases in sedimentary δ15N occurred at 1835 AD and 1965 AD, and variance increased following the 1965 AD shift. The largest of these abrupt shifts at 1835 AD coincided, within age model error, with an abrupt increase in marine productivity indicators in sediment records off the coast of Perú and Chile. In the latter part of the twentieth century, δ15N values increased during periods of higher landings of Peruvian anchoveta and sardines. We hypothesise that seabird presence and activity on Genovesa increased during periods of higher regional marine productivity. Enhanced variance in Genovesa δ15N following the 1965 AD shift may reflect a modern population more susceptible to climate and environmental variability than at any other time in the last 400 years.

Glacial-to-interglacial changes in nitrate supply and consumption in the subarctic North Pacific from microfossil-bound N isotopes at two trophic levels

Reduced nitrate supply to the subarctic North Pacific (SNP) surface during the last ice age has been inferred from coupled changes in diatom-bound δ15N (DB-δ15N), bulk sedimentary δ15N, and biogenic fluxes. However, the reliability of bulk sedimentary and DB-δ15N has been questioned, and a previously reported δ15N minimum during Heinrich Stadial 1 (HS1) has proven difficult to explain. In a core from the western SNP, we report the foraminifera-bound δ15N (FB-δ15N) in Neogloboquadrina pachyderma and Globigerina bulloides, comparing them with DB-δ15N in the same core over the past 25 kyr. The δ15N of all recorders is higher during the Last Glacial Maximum (LGM) than in the Holocene, indicating more complete nitrate consumption. N. pachyderma FB-δ15N is similar to DB-δ15N in the Holocene but 2.2‰ higher during the LGM. This difference suggests a greater sensitivity of FB-δ15N to changes in summertime nitrate drawdown and δ15N rise, consistent with a lag of the foraminifera relative to diatoms in reaching their summertime production peak in this highly seasonal environment. Unlike DB-δ15N, FB-δ15N does not decrease from the LGM into HS1, which supports a previous suggestion that the HS1 DB-δ15N minimum is due to contamination by sponge spicules. FB-δ15N drops in the latter half of the Bolling/Allerod warm period and rises briefly in the Younger Dryas cold period, followed by a decline into the mid-Holocene. The FB-δ15N records suggest that the coupling among cold climate, reduced nitrate supply, and more complete nitrate consumption that characterized the LGM also applied to the deglacial cold events.

Ecosystem development following deglaciation: A new sedimentary record from Devils Lake, Wisconsin, USA

Processes and rates of ecosystem development can be reconstructed using lacustrine sedimentary sequences, but this approach often requires records that contain the start of primary succession. Most lakes in the upper Midwestern U.S. were formed by glaciers at the end of the last Ice Age approximately 11,700 cal yr BP. Devils Lake, Wisconsin is a rare example of a lake from this region whose sediments extend into the Pleistocene and may include the Last Glacial Maximum. Sediment magnetic, geochemical, pollen, and charcoal records were generated from a 10 m core whose basal sediments may be 28,000 years old. Together with a previously published pollen record, these proxies combine to reveal a history of long-term climatic, vegetative and geologic change during the late Pleistocene to Holocene. We identify six sedimentary units that indicate a series of consecutive events rather than a predictable trajectory of ecosystem development at the site. Productivity in the lake was low during the late Pleistocene and increased during the Holocene, as reflected by the sediment lithology, which shows a sudden shift from glacial vivianite-rich and organic-poor clastic-dominated sediments to Holocene diatomaceous sapropels. Several important processes initiated around 17,000 cal yr BP, including the onset of organic matter accumulation and fire in the terrestrial ecosystem. However, the post-glacial landscape was not devoid of vegetation because pollen assemblages indicate that terrestrial vegetation, likely a spruce tundra, survived near the site. A switch to a hardwood forest period during the Holocene also led to a change in the fire regime, with increased frequency of burning. Aquatic ecosystem productivity lagged terrestrial ecosystem productivity throughout the record. Nutrient cycling (as recorded by sedimentary δ15N) was variable but not directional, and appeared to be correlated with climate conditions early in the record, and terrestrial ecosystem processes later in the record. Throughout the Holocene magnetic mineral concentration decreased as productivity increased, and the source of magnetic material shifted from almost exclusively lithogenic to approximately 50% derived from soil or biogenic sources. Magnetic grain size, Ambrosia pollen percentages, and charcoal concentration increased and δ15N decreased in the most recent part of the record, due to anthropogenic influence in the region including agricultural activities.

The Comparison of δ13C Values of a Deposit- and a Suspension-Feeder Bio-Indicates Benthic vs. Pelagic Couplings and Trophic Status in Contrasted Coastal Ecosystems

δ13C and δ15N values of two generalists primary consumers, a strict deposit-feeder polychaete (Arenicola marina), and a strict suspension-feeder bivalve (Crassostrea gigas), were investigated to typify the trophic functioning of two contrasted marine coastal ecosystems (eutrophic and mesotrophic, east and west Cotentin peninsula, respectively, English Channel, Normandy, France). On average, δ13C and δ15N values of lugworms mirrored those of sediment organic matter (SOM), whereas δ13C and δ15N of oysters mirrored those of suspended particulate organic matter (SPOM). δ13C values of the two species displayed significant differences on the west coast (mesotrophic) contrary to the east coast (eutrophic; significant interactions). δ15N values differed only between sites and not between species. Diet of A. marina relied exclusively on microphytobenthos (MPB) and detritus of macroalgae (ULV) on the mesotrophic coast, whereas diet of C. gigas relied mainly on SPOM. Conversely, on the eutrophic ecosystem (the east coast), both species displayed the same diet, which was a mixture of pelagic sources (SPOM), benthic sources (MPB and ULV) and to a lesser extent riverine particulate organic matter (rPOM). These results were explained by the intensity of benthic vs. pelagic couplings (i.e. benthic-pelagic and pelagic-benthic) which differed in the two ecosystems. Low trophic coupling occurred on the mesotrophic (west) coast, whereas benthic-pelagic (SOM resuspension) and pelagic-benthic (settling of SPOM such as phytoplankton blooms) couplings were typified on the eutrophic (east) coast. This higher particulate organic matter (POM) pelagic-benthic coupling on the east coast was probably enhanced by nutrient enrichment caused by eutrophication. Comparison of δ13C ratios of both the strict deposit-feeder (e.g. A. marina) and the strict suspension-feeder (e.g. C. gigas) was then proposed as a bio-indicator of the trophic status and of POM benthic vs. pelagic couplings of soft-bottom coastal ecosystems.