Sediment Trap Material Research Articles

Seasonal changes in molecular composition of organic matter in lake sediment trap material from Nylandssjön, Sweden

The origin and degradation/preservation state of organic matter (OM) in lacustrine systems can be studied using its molecular composition. Pyrolysis gas chromatography–mass spectrometry (Py–GC–MS) allows rapid assessment of OM-rich samples and has proven a useful tool for peat, lacustrine and marine deposits, but is rarely applied specifically to lake environments prone to the formation of varves (annually laminated sediments), which are used frequently for palaeoenvironmental reconstruction. We applied Py–GC–MS to a 3.5yr record of sediment trap samples collected from Nylandssjön in northern Sweden to determine the usefulness of the method for improving knowledge of OM dynamics in varved lakes. It appeared that seasonal variation controlled the pyrolysis fingerprints: intact polysaccharides and lignin from vascular plants, chitin from arthropods and chlorophyll from algae were concentrated in spring and summer traps, whereas winter trap samples were enriched mainly in degraded components, as OM slowly trickled down under a thick layer of ice. A secondary diagenetic process involving sulfurisation of phytadienes was recognised via isoprenoid thiophenes, with no clear seasonal trend, probably because it occurred under anoxic conditions at the lake bottom. The N-containing compounds from proteins in algae and chitin in arthropods were abundant in all samples, indicating that the dominant flux of varve OM was autochthonous in origin. We conclude that Py–GC–MS has significant potential for identifying the variability in parent material (source organisms and degradation state) and could become a valuable tool for palaeoenvironmental reconstruction from varved lake sediments.

Transfer of organic carbon through marine water columns to sediments – insights from stable and radiocarbon isotopes of lipid biomarkers

Abstract. Compound-specific 13C and 14C compositions of diverse lipid biomarkers (fatty acids, alkenones, hydrocarbons, sterols and fatty alcohols) were measured in sinking particulate matter collected in sediment traps and from underlying surface sediments in the Black Sea, the Arabian Sea and the Ross Sea. The goal was to develop a multiparameter approach to constrain relative inputs of organic carbon (OC) from marine biomass, terrigenous vascular-plant and relict-kerogen sources. Using an isotope mass balance, we calculate that marine biomass in sediment trap material from the Black Sea and Arabian Sea accounted for 66–100% of OC, with lower terrigenous (3–8%) and relict (4–16%) contributions. Marine biomass in sediments constituted lower proportions of OC (66–90%), with consequentially higher proportions of terrigenous and relict carbon (3–17 and 7–13%, respectively). Ross Sea data were insufficient to allow similar mass balance calculations. These results suggest that, whereas particulate organic carbon is overwhelmingly marine in origin, pre-aged allochthonous terrigenous and relict OC become proportionally more important in sediments, consistent with pre-aged OC being better preserved during vertical transport to and burial at the seafloor than the upper-ocean-derived marine OC.

Impact of sedimentary degradation and deep water column production on GDGT abundance and distribution in surface sediments in the Arabian Sea: Implications for the TEX86 paleothermometer

The TEX86 is a widely used paleotemperature proxy based on isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs) produced by Thaumarchaeota. Archaeal membranes are composed of GDGTs with polar head groups (IPL-GDGTs), most of which are expected to be degraded completely or transformed into more recalcitrant core lipid (CL)-GDGTs upon cell lysis. Here, we examined the differences in concentration and distribution of core lipid (CL)- and intact polar lipid (IPL)-GDGTs in surface sediments at different deposition depths, and different oxygen bottom water concentrations (<3–83μmolL−1). Surface sediments were sampled from 900 to 3000m depth on a seamount (Murray Ridge), whose summit protrudes into the oxygen minimum zone of the Arabian Sea. Concentrations of organic carbon, IPL- and CL-GDGTs decreased linearly with increasing maximum residence time in the oxic zone of the sediment (tOZ), suggesting increasing sedimentary degradation of organic matter and GDGTs. IPL-GDGT-0 was the only exception and increased with tOZ, indicating that this GDGT was probably produced in situ in the surface sediment. Concentrations of crenarchaeol with glycosidic headgroups decreased with increasing tOZ, while crenarchaeol with a hexose, phosphohexose head (HPH) group, in contrast, showed an increase with increasing tOZ, indicating that the concentration of HPH crenarchaeol was primarily determined by in situ production in surficial sediments. TEX86 values of both IPL-derived GDGTs and CL-GDGTs decreased by ∼0.08units with increasing water depth, in spite of the sea surface temperatures being identical for the restricted area studied. In situ production in sediments could be excluded as the main cause, due to the slow production rates of GDGTs in sediments, and previous observations of the same trends in TEX86 in sediment trap material. Instead, the incorporation of GDGTs produced in the oxygen minimum zone (with high TEX86 values) and their preferential degradation during the sinking through the water column, or differential degradation of IPL-GDGTs per head group could be the causes for the observed change in TEX86 values. The effect of differential degradation might cause differences between oxic and anoxically deposited sediments, and, together with a potential deep water contribution on TEX86 values, could translate into changes in reconstructed temperature of <3°C, which might have to be accounted for in TEX86 calibration and paleotemperature studies of deep water sedimentary records.

PCDD/Fs, PCBs and PBDEs in zooplankton in the Baltic Sea – Spatial and temporal shifts in the congener-specific concentrations

In the marine food-webs, zooplankton is a key element in the transfer of persistent organic pollutants to higher trophic levels. We determined the congener-specific concentrations of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) in offshore zooplankton (size from 0.2 to 20mm) collected in 2001, 2002 and 2010 in the northern and central Baltic Sea. Of the PCDD/Fs, the concentrations of 2378-TCDF were from 18 to 47 and of 23478-PeCDF from 7.9 to 29pgg−1fat and showed little temporal differences. However, 1234678-HpCDF and OCDF were abundant in 2001–2002 especially in the eastern Gulf of Finland (average concentrations 50 and 89pgg−1fat, respectively). In 2010 the concentrations of these two congeners were lower, 29 and 30pgg−1fat, respectively, but still substantially higher than in the other surveyed areas. The principal components analysis (PCA) supported that area-specific patterns in pollution strongly contributed to the congener profiles particularly in surface sediment and in sediment trap material, but even in zooplankton. The concentrations of the PCBs were highest in the Gulf of Finland and in the Bothnian Bay. The concentrations of most PCBs were somewhat lower in 2010 than in 2001–2002. Of the dioxin-like PCBs, the concentrations of PCB-77 were highest (271–572pgg−1fat) but PCB-126 (32–113pgg−1fat) contributed from 85% to 91% of the total toxicity of PCBs due to its higher toxic potency. Of the PBDEs, the BDE47 and BDE99 were the most abundant (concentrations from 1.2 to 4.6 and from 0.4 to 3.3ngg−1 fat, respectively). The concentrations of most PBDEs were lower in 2010 than in 2001/2002 except in the eastern Gulf of Finland.

Sediment resuspension in the Lake Erie nearshore

A decline in water quality in Lake Erie during the last decade, despite increased efforts to limit nutrient loading, may be better understood by examining internal processes in the lake. We employed 7Be, 210Pb and 137Cs measurements of suspended matter in tributaries, in the lake water column, in atmospheric precipitation, in sediment traps and in bottom sediments collected in June and August/September 2011 to estimate the fraction of the suspended matter that is resuspended from the bottom. Mass balances on 7Be and 210Pb using sediment trap material indicated that at the nearshore site ~83–94% of suspended matter in the water column was resuspended bottom sediment, while, offshore, resuspended sediment made up only ~62–75%. A mass balance using the 7Be/210Pb ratio for each sediment source indicated that resuspension of bottom sediment accounted for 52–97% of the suspended material in the nearshore and from 53 to 86% of the suspended matter in the offshore and was greater after the fall overturn. The amount of nutrients delivered to the water column by resuspension indicates that the resuspension loading of particle-bound P to the lake is about the same as the tributary loading, although the resuspended P is likely to be significantly less bioavailable.

A role for nitrite in the production of nitrous oxide in the lower euphotic zone of the oligotrophic North Pacific Ocean

Understanding the role of the oceans in the Earth's changing climate requires comprehension of the relevant metabolic pathways which produce climatically important trace gases. The global ocean represents one of the largest natural sources of nitrous oxide (N2O) that is produced by selected archaea and/or bacteria during nitrogen (N) metabolism. In this study, the role of nitrite (NO2−) in the production of N2O in the upper water column of the oligotrophic North Pacific Subtropical Gyre was investigated, focusing primarily on the lower euphotic zone where NO2− concentrations at the primary NO2− maximum reached 195nmolL−1. Free-drifting sediment trap arrays were deployed to measure N cycle processes in sinking particulate material and the addition of selected N substrates to unpreserved sediment traps provided an experimental framework to test hypotheses regarding N2O production pathways and controls. Sinking particles collected using NO2−-amended, unpreserved sediment traps exhibited significant production of N2O at depths between 100 and 200m. Subsequent stable isotope tracer measurements conducted on sediment trap material amended with 15NO2− yielded elevated δ15N values of N2O, supporting N2O production via a NO2− metabolism pathway. Experiments on seawater collected from 150m showed N2O production via NO2− metabolism also occurs in the water-column and indicated that the concentration of NO2− relative to NH4+ availability may be an important control. These findings provide evidence for the production of N2O via nitrifer-denitrification in the lower euphotic zone of the open ocean, whereby NO2− is reduced to N2O by ammonia-oxidizing microorganisms.

Sedimentation of PCDD/Fs and PCBs in the Gulf of Finland and the Gulf of Bothnia, the Baltic Sea

Sediment trap material was collected during May–December in the period 1996–2008 in three coastal areas and four open sea stations in the Finnish territory of the Baltic Sea. The highest sedimentation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) with a typical source-related congener profile from chlorophenol production dominated by highly chlorinated dibenzofurans was found close to a historical source in the Kymijoki estuary. This was an order of magnitude higher than in other river estuaries and two orders of magnitude higher than in the open sea stations. The sedimentation of polychlorinated biphenyls (PCBs) was also higher in river estuaries than in other areas. No significant decrease over a 12year period of monitoring was found in concentration or in sedimentation in the Kymijoki estuary. In the western Gulf of Finland, the Archipelago Sea and the Gulf of Bothnia, the dominating congeners, calculated as toxic equivalent (TEQ) in sedimentation were 1, 2, 3, 7, 8-PeCDD and 2, 3, 4, 7, 8-PeCDF, often reported as the main congeners in deposition.

Major contribution of diatom resting spores to vertical flux in the sub-polar North Atlantic

The mass sinking of phytoplankton cells following blooms is an important source of carbon to the ocean's interior, with some species contributing more to the flux of particulate organic carbon (POC) than others. During the 2008 North Atlantic Bloom Experiment in the Iceland Basin, we examined plankton community composition from surface waters and from sediment traps at depths down to 750m. Samples collected with neutrally buoyant Lagrangian sediment traps captured a major flux event. Diatoms comprised ≥99% of cell flux into the sediment traps, with vegetative cells and resting spores of the genus Chaetoceros contributing 50–95% of cell flux. Resting spores of one species, identified as Chaetoceros aff. diadema, were dominant, comprising 35–92% of cell flux. The flux of resting spores ranged from 2 to 63mgCm−2day−1 and was significantly correlated with POC flux (p=0.003). Over the course of 10 days, the flux of resting spores increased by 26 fold, suggesting that the cells sank en masse, possibly in aggregates. In contrast, vegetative cells of C. aff. diadema sampled from surface waters during the period preceding the flux event generally comprised <1% of the diatom community and never exceeded 5.2%. Resting spores of C. aff. diadema were rarely observed in surface waters but their concentrations increased with depth (to 200m) below the mixed layer. This increase in resting spore abundance, coupled with increased dissolved silicic acid concentrations at depth, suggest that the morphological changes associated with spore formation may have occurred in the mesopelagic zone, while cells were sinking. The values of variable fluorescence (Fv/Fm) measured on sediment trap material dominated by resting spores were among the highest values measured in the study area at any depth. This, in combination with the rapid germination of resting spores in ship-board incubations, suggests that vegetative cells were not physiologically stressed during spore formation. The degradation-resistant, heavily silicified resting spore valves explain the high relative contribution of C. aff. diadema resting spores to total plankton carbon at depth. These data emphasize the ephemeral nature of organic carbon flux events in the open ocean and highlight how non-dominant species and transient life stages can contribute more to carbon flux than their more abundant counterparts.

Amino acid nitrogen isotopic composition patterns in lacustrine sedimenting matter

Amino acids (AAs) comprise a large fraction of organic nitrogen (N) in plankton and sedimenting matter. Aquatic studies of organic N compounds in general and of AAs in particular, mostly concentrate on marine environments. In order to study the cycling and fate of organic N and AAs in lakes, we measured the N isotopic composition (δ15N) of bulk organic matter (OM) and of single hydrolysable AAs in sediment trap and sediment samples from two Swiss lakes with contrasting trophic state: Lake Brienz, an oligotrophic lake with an oxic water column, and Lake Zug a eutrophic, meromictic lake. We also measured the N isotopic composition of water column nitrate, the likely inorganic N source during biosynthesis in both lakes. The δ15N–AA patterns found for the sediment trap material were consistent with published δ15N–AA data for marine plankton. The AA composition and primary δ15N–AA signatures are preserved until burial in the sediments. During early sedimentary diagenesis, the δ15N values of single AAs appear to increase, exceeding those of the bulk OM. This increase in δ15N–AA is paralleled by a decreased contribution of AAs to the total OM pool with progressed degradation, suggesting preferential AA degradation associated with a significant N isotope fractionation. Indicators for trophic level based on δ15N–AAs were determined, for the first time in lacustrine systems. In our samples, the trophic AAs were generally enriched in 15N compared to source AAs and higher trophic δ15N–AA values in Lake Zug were consistent with a higher trophic level of the bulk biomass compared to Lake Brienz. Especially the difference between average trophic δ15N–AAs and average source δ15N–AAs was sensitive to the trophic states of the two lakes. A proxy for total heterotrophic AA re-synthesis (ΣV), which is strongly associated with heterotrophic microbial reworking of the OM, was calculated based on δ15N values of trophic AAs. Higher ΣV in Lake Brienz indicate enhanced heterotrophic bacterial reworking of AAs under oligotrophic conditions. Despite changes in the δ15N–AA values within the sediments, the proxies based on these values were consistent over the studied sediment profile, indicating the preservation of trophic signatures; therefore, our results underscore that δ15N–AA analysis of sedimentary records represents a promising tool to assess trophic levels and bacterial re-synthesis in lakes.

Downward fluxes of elemental carbon, metals and polycyclic aromatic hydrocarbons in settling particles from the deep Ionian Sea (NESTOR site), Eastern Mediterranean

Abstract. To assess sources and major processes controlling the vertical transport of both anthropogenic and natural chemical species in a deep basin of the Eastern Mediterranean Sea (SE Ionian Sea, Nestor site), we performed chemical characterization (elemental carbon, major and trace metals and polycyclic aromatic hydrocarbons) of marine sinking particles. Sediment traps were deployed at five successive depths, 700, 1200, 2000, 3200 and 4300 m from the sea surface from May 2007 to October 2008. Fluxes of all measured chemical species, attributed to both natural and anthropogenic sources, exhibited minimum values from January to March 2008 and maximum from April to September 2008. Crustal matter flux from atmospheric inputs – either "freshly" deposited or stored in the surface layers – plays an important role in the temporal variability of particulate marine matter fluxes along with particulate organic carbon export, imposing ballast effects. Tracers (elemental carbon, retene) of the devastating forest fires that occurred in August 2007 in southern Greece were detected in sediment trap material from all depths with a delay of 15 days at 4300 m, indicating a rapid and well-coupled transport of sinking particulate material between the sea-surface and deep layers of the Eastern Mediterranean Sea. This is in accordance with an evident covariance between certain compounds, suggesting common sources and/or transport mechanisms to depth. Lateral inputs of pollutants at the deepest trap (4300 m) are probably of importance, related to the periodic influence of deep Adriatic water at the study site.

Comparison of TEX86 and [formula omitted] temperature proxies in sinking particles in the Cariaco Basin

The Cariaco Basin, a silled, permanently anoxic basin on the continental shelf of Venezuela with a dynamic chemocline (~240–350m), has been subject of >20 years of oceanographic observation and sediment trap studies. We evaluated U37K′ and the TEX86 temperature proxies using sinking particles collected in shallow sediment trap samples at 275m (Trap A) and 455m (Trap B) (within and below the chemocline). The organic geochemical temperature proxies, U37K′ (based on coccolithophorid alkenone lipids) and TEX86 (based on archaeal glycerol dialkyl glycerol tetraether (GDGT) lipids), use observed relationships between the ratio of specific lipids and measured sea surface temperature to hindcast past sea surface temperatures. In this study, both U37K′ and TEX86 temperature proxies record seasonal temperature variations, including the cooling associated with upwelling events. U37K′-based temperatures are colder than measured sea surface temperatures, and better correlated temperature at the chlorophyll maximum. In sediment trap material collected below the chemocline (Trap B),U37K′ values are higher than those in Trap A. Warmer subchemocline U37K′ based temperatures may be related to autooxidation of sinking particles, either by small amounts of available oxygen or by alternate electron acceptors concentrated in the biologically dynamic chemocline (e.g. intermediate sulfur compounds). The absolute flux weighted TEX86 temperature values measured in sinking particles from Trap A match the measured SST well. The differences in the TEX86 values between Traps A and B are small and reflect less impact of degradation. Overall, the TEX86 temperatures in sinking particles in the Cariaco Basin reflect annual SST.

Diatom taphonomy and silica cycling in two freshwater lakes and their implications for inferring past lake productivity

Diatom preservation can be a major taphonomic issue in many lakes but is often unrecognised and its impacts on qualitative and quantitative inferences (such as productivity and biodiversity estimates) from sedimentary archives are seldom explored. Here two palaeolimnological case studies of 20th-century anthropogenic eutrophication of freshwater lakes in Northern Ireland (Lough Neagh and Lough Augher) are re-visited and new data presented on diatom preservation. Assessing problems of taphonomy challenges previous interpretations of silica dynamics and diatom productivity at these sites. Diatom preservation was assessed in both sediment trap material and sediment cores from Lough Neagh, and in sediment cores from Lough Augher. Preservation data, combined with geochemical analysis (Si, Fe), provide an insight into silica cycling and diatom accumulation over a range of temporal scales from these lakes. Diatom preservation was generally good for the Lough Neagh material, although differential (better) preservation of the smaller Aulacoseira subarctica compared to the larger Stephanodiscus neoastraea sensu lato valves was clear, especially in sediments. Porewater silica showed a complex seasonal pattern in the upper sediment, against expectations of steady-state. The Lough Augher material was generally poorly preserved, although preservation (dissolution) was significantly (and positively) correlated to bulk sedimentation rate, and was found to be a major control on (net) diatom accumulation rate across the basin. Past seasonal and severe anoxia at Lough Augher did not improve diatom preservation, contrary to some previous studies, which may be due to extreme changes in sedimentary redox conditions. Finally, using published experimental relationships between dissolution and diatom valve loss, correction factors were applied to previously published profiles of diatom accumulation over the last ~150 years (biovolume from Lough Neagh and frustule accumulation rate from Lough Augher), which suggest that diatom productivity estimates from sedimentary records are underestimated by a factor of 2–4 due to dissolution effects alone. The results clearly have implications for the reliability and accuracy of diatom-based inferences made from sediment records, both qualitative and quantitative, especially for those that employ diatoms as direct measures of productivity or biodiversity.

An approach for particle sinking velocity measurements in the 3–400 μm size range and considerations on the effect of temperature on sinking rates

The flux of organic particles below the mixed layer is one major pathway of carbon from the surface into the deep ocean. The magnitude of this export flux depends on two major processes—remineralization rates and sinking velocities. Here, we present an efficient method to measure sinking velocities of particles in the size range from approximately 3–400 μm by means of video microscopy (FlowCAM®). The method allows rapid measurement and automated analysis of mixed samples and was tested with polystyrene beads, different phytoplankton species, and sediment trap material. Sinking velocities of polystyrene beads were close to theoretical values calculated from Stokes’ Law. Sinking velocities of the investigated phytoplankton species were in reasonable agreement with published literature values and sinking velocities of material collected in sediment trap increased with particle size. Temperature had a strong effect on sinking velocities due to its influence on seawater viscosity and density. An increase in 9 °C led to a measured increase in sinking velocities of ~40 %. According to this temperature effect, an average temperature increase in 2 °C as projected for the sea surface by the end of this century could increase sinking velocities by about 6 % which might have feedbacks on carbon export into the deep ocean.

Reconstructing calcification in ancient coccolithophores: Individual coccolith weight and morphology of Coccolithus pelagicus (sensu lato)

We have adapted an existing method to estimate coccolith calcite weight using birefringence (Beaufort, 2005) to suit the large coccoliths of Coccolithus pelagicus, which are only partially birefringent under cross-polarised light microscopy. Fossil and sediment trap material from the South Tasman Rise region of the Southern Ocean was used for calibration and validation. Our approach was tested with only the coccolith central area (CA) considered for measurement, to avoid relying on the less robust proximal shields. Thus our results are relative and intend to quantify intra-specific variations in volumetric calcite weight, expressed as a Weight Index (WI). Our results were overall consistent with mass estimation based on distal shield lengths. However, the WI approach clearly has the advantage in exploring allometric scaling between coccolith size and weight, as well as in measuring the degree of calcification in similarly sized morphotypes. Combining WI and morphometry data (distal shield length, DSL), we demonstrate subtle, but statistically significant changes in shape and thus calcification degree both within and between the tested Coccolithus populations. Most strikingly, it appears that modern Coccolithus populations in the Southern Ocean are, on average, more heavily calcified than their fossil counterparts.

Fifteen year time-series of radiolarian fluxes and environmental conditions in the Bering Sea and the central subarctic Pacific, 1990–2005

In order to examine radiolarian fluxes and evaluate their relationships to environmental conditions, a fifteen year long time-series observation on radiolarian fluxes was conducted using sediment trap materials collected at the Bering Sea Station AB (53.5°N, 177°W) and the central subarctic Pacific Station SA (49°N, 174°W) during 1990–2005. Encountered radiolarian assemblages include 124 taxa at Station AB and 110 taxa at Station SA. In general, total radiolarian fluxes of the coarse fraction (63μm−1mm) at Station AB (mean: 8.7×103shellsm−2d−1) were greater than those observed at Station SA (mean: 6.5×103shellsm−2d−1), mainly due to the differences in physico-chemical conditions (i.e., mixed layer depth, surface circulation, nutrient concentrations, etc.). Out of >110 taxa, thirty-eight abundant radiolarian taxa were chosen for enumeration and further statistical treatment, including the taxa with >1% annual mean occurrence. As abundant radiolarian species display variable flux patterns during the study period, an R-mode cluster analysis was used to identify eight clusters at Station AB, and eleven clusters at Station SA. Marked changes in the relative contribution of each of the clusters to the total radiolarian assemblages are apparent. Notably high flux maxima of total radiolarians were observed around 1990, 1995, 2000, 2003 and 2004. The observed inter-annual variations in the faunal assemblages in the Bering Sea and the central subarctic Pacific were related to large scale climatic shift such as the Arctic Oscillation (AO). The negative correlation between the radiolarian assemblages including surface to subsurface dwellers and the AO Index may suggest a significant influence by the intensity of surface water mixing and water mass exchanges.

Direct evidence for the alteration of13C natural abundances during early diagenesis in Lake Kasumigaura, Japan

In sediment trap materials or suspended particulate organic matter, δ13C values must be averaged over at least several years to determine δ13C shifts during sedimentation. To overcome this problem, we studied isotopic modification of sedimentary organic carbon during early diagenesis in sediment samples collected intermittently from a site at the center of Lake Kasumigaura, Japan, over a period of nearly 30 years, beginning in 1979. We examined the degree of apparent isotope discrimination by comparing long-term changes in the carbon isotope composition of the surface sediment (top 2 cm) with the isotope composition profile from 0 to 15 cm. The downcore C isotope composition profile was close to the values calculated from the long-term changes in the isotope composition of the surface sediment, suggesting that the sedimentary carbon isotope composition was generally conserved (±0.6‰) during early diagenesis (up to 30 years). However, at greater core depths, significant 13C enrichment of up to 1.5‰ was sometimes observed, especially in the periods of relatively oxidative conditions.

Nitrogen isotopic relationship between diatom‐bound and bulk organic matter of cultured polar diatoms

Using batch cultures, the 15N/14N (hereafter δ15N) of diatom‐bound organic matter was measured and compared to the δ15N of total diatom biomass during the progressive consumption of a nitrate pool in four polar diatom species (Fragilariopsis cylindrus, Fragilariopsis kerguelensis, Pseudo‐nitzschia seriata, and Thalassiosira nordenskioeldii) and one temperate species (Thalassiosira aestivalis). In general, the δ15N of the dissolved nitrate in seawater was greater than that of the biomass, which was greater than that of the diatom‐bound N. Rayleigh‐type relationships were observed, allowing for estimation of the isotope effect (ɛ) for each species, with a range from 1.0‰ to 14.0‰ across all species. For all cultured strains, the δ15N values of the diatom‐bound (δ15NDB) fraction was lower than those of the total diatom biomass (δ15Nbiomass). The isotopic offset between the biomass and diatom‐bound N (δ15NDBoffset = δ15Nbiomass − δ15NDB) was relatively constant along the growth curve for each individual species but varied among species, with a range of 1.9‰–11.2‰. Weak relationships were determined when ɛ and the δ15NDBoffset were compared to cellular size and surface area:volume ratio. More significantly, with the exception of Pseudo‐nitzschia seriata, a strong positive relationship was found between ɛ and δ15NDBoffset. While the culture data indicate a positive δ15NDBoffset across all studied diatom species, surface sediment data suggest a negative δ15NDBoffset for sedimentary assemblages. This indicates that either (1) the growth conditions of our cultures had some effect on δ15NDBoffset or (2) a low‐δ15N component of the N that we measure as diatom frustule–bound is lost during early diagenesis. Given documented assemblage changes, our culture data for relevant species do not suggest that the higher δ15NDB observed in the Antarctic during ice ages can be explained by species related changes in the sedimentary bulk‐to‐diatom‐bound isotopic difference. Future work on the diatom‐bound material in cultured diatoms grown under in situ nutrient conditions, analysis of sediment trap and net tow material, and frustule dissolution experiments will more completely assess this paleoproxy.

Pseudo-nitzschia and domoic acid fluxes in Santa Barbara Basin (CA) from 1993 to 2008

Blooms of domoic acid (DA) producing Pseudo-nitzschia, regularly occur off the coast of California. Although it has been hypothesized that these blooms are increasing in frequency, the lack of historical records limits our understanding of potential causal mechanisms. In this study, an 15-year time-series (1993–2008) of sediment trap samples collected from the Santa Barbara Basin (SBB) at 540 m were analyzed for Pseudo-nitzschia ( n = 196, microscopy and SEM) and DA ( n = 206, LC–MS/MS) concentrations and fluxes. Results suggest that there was an abrupt shift towards greater frequency and higher magnitude Pseudo-nitzschia blooms and toxic DA flux events in the SBB after the year 2000. SEM analysis of sediment trap material indicates that these events were mainly blooms of P. australis, with cell fluxes increasing by an order of magnitude from a maximum of 4.5 × 10 6 cells m −2 d −1 pre-2000, to as high as 3.2 × 10 8 cells m −2 d −1 thereafter. Similarly, sediment trap DA fluxes increased by an average of 13.4 μg m −2 d −1, with only one large event (>5 μg m −2 d −1) from 1993 to 1999 versus 16 large DA events from 2000 to 2008. While the causes of this abrupt shift remain ambiguous, we suggest that this shift may be related to natural climate variability associated with a change in phase of the North Pacific Gyre Oscillation (NPGO) and its potential influence on the composition and magnitude of waters that are upwelled into the SBB.

Particulate sulfur species in the water column of the Cariaco Basin

The biogeochemistry of iron sulfide minerals in the water column of the Cariaco Basin was investigated in November 2007 (non-upwelling season) and May 2008 (upwelling season) as part of the on-going CARIACO (CArbon Retention In A Colored Ocean) time series project. The concentrations of particulate sulfur species, specifically acid volatile sulfur (AVS), greigite, pyrite, and particulate elemental sulfur, were determined at high resolution near the O 2/H 2S interface. In November 2007, AVS was low throughout the water column, with the highest concentration at the depth where sulfide was first detected (260 m) and with a second peak at 500 m. Greigite, pyrite, and particulate elemental sulfur showed distinct concentration maxima near the interface. In May 2008, AVS was not detected in the water column. Maxima for greigite, pyrite, and particulate elemental sulfur were again observed near the interface. We also studied the iron sulfide flux using sediment trap materials collected at the Cariaco station. Pyrite comprised 0.2–0.4% of the total particulate flux in the anoxic water column, with a flux of 0.5–1.6 mg S m −2 d −1. Consistent with the water column concentration profiles for iron sulfide minerals, the sulfur isotope composition of particulate sulfur found in deep anoxic traps was similar to that of dissolved sulfide near the O 2/H 2S interface. We conclude that pyrite is formed mainly within the redoxcline where sulfur cycling imparts a distinct isotopic signature compared to dissolved sulfide in the deep anoxic water. This conclusion is consistent with our previous study of sulfur species and chemoautotrophic production, which suggests that reaction of sulfide with reactive iron is an important pathway for sulfide oxidation and sulfur intermediate formation near the interface. Pyrite and elemental sulfur distributions favor a pathway of pyrite formation via the reaction of FeS with polysulfides or particulate elemental sulfur near the interface. A comparison of thermodynamic predictions with actual concentration profiles for iron sulfides leads us to argue that microbes may mediate this precipitation.

The association between biogenic and inorganic minerals and the amino acid composition of settling particles

To test the hypothesis that calcium carbonate (rather than opal) carries most organic carbon to the deep sea, total hydrolysable amino acid (THAA) analysis was applied to deep‐sea (3000 m) sediment trap material from the northeast Atlantic (PAP Site), a variable but intrinsically carbonate‐dominated system. THAAs were analyzed in conjunction with total organic carbon, biogenic silica, calcium carbonate, and inferred lithogenic fluxes. The THAA‐based degradation state of organic carbon could not be systematically explained by changes in the flux of different mineral phases, which could account for only 16% of the observed variability. In addition amino acid parameters indicative of source organisms indicate that diatom cell walls are an important residual component of organic carbon reaching the deep ocean, a finding supported by comparison with data from previous studies of diverse oceanic environments. Finally, during 2001, very high organic carbon fluxes were associated with elevated lithogenic fluxes and low organic matter degradation relative to surrounding years. In accordance with other recent experimental and observational studies, the data indicate that under specific export scenarios, lithogenic fluxes can act as highly significant mediators of organic carbon transfer to the deep ocean.