Water Column Production Research Articles

Ecological responses to solar forcing during the Homerian Climate Anomaly recorded by varved sediments from Holzmaar, Germany

Features like solar cyclicities and trends as well as grand solar minima are used to attribute natural climate variability to solar forcing on decadal to millennial time scales. Here we focus on ecological responses of a Grand Solar Minimum on annually- laminated lake sediments from Holzmaar (Germany) covering the Homeric Climate Anomaly (HCA). Diatom assemblages and pigments of purple sulphur bacteria (Bphe a) analysed at decadal resolution document well-stratified conditions with relatively low lacustrine productivity prior to the HCA (2950–2750 cal. BP). Colder temperatures, a well-mixed water column and higher primary aquatic productivity established during the HCA (2750–2680 cal. BP) as indicated for Holzmaar by dominance of the planktonic diatom Stephanodiscus minutulus, decreasing Bphe a and peaking total chloropigment concentrations. The termination of the HCA after 2680 cal. BP is marked by additional anthropogenic signals related to deforestation that changed the catchment at the contemporaneous Bronze Age/Iron Age transition. Our high-resolution and well-dated multiproxy study based on varved sediments contributes to a better understanding of decadal-scale responses of aquatic ecosystems to solar forcing and compares well with hypotheses suggested by other investigations indicating colder and more windy climatic conditions as the consequences of a Grand Solar Minimum for mid-latitudes of the Northern Hemisphere.

Aquacultural source of nitrous oxide revealed by nitrogen isotopes

The rapid expansion of coastal aquaculture has led to an increase in the coverage of aquaculture ponds, where intense feed-derived nitrogen is causing significant emissions of nitrous oxide (N2O). Multiple N2O production pathways and the relative importance of water column vs. sedimentary production in aquaculture ponds remain uncertain. Clarifying these pathways is vital for sustainable aquaculture development. Using 15N-labeled dissolved inorganic nitrogen, the pathways and rates of N2O production in subtropical aquaculture ponds located in south China, cultivating whiteleg shrimp, Japanese seabass, and giant river prawn, were successfully characterized. Total N2O production rates ranged from 6 to 70 µmol-N m−2 d−1, with the shrimp pond exhibiting the highest total N2O production rates, followed by ponds for seabass and prawn. These differences are primarily due to varying feed amounts causing differences in dissolved nutrients in water column and sediment. Particularly, nutrient and organic matter accumulation at the surface sediment stimulated N2O production. The oxygenated sediment on a centimeter scale could produce substantially more N2O compared to the water column above on a meter scale. Partial denitrification, i.e., nitrate and nitrite reduction to N2O, was more important (> 60 %) for N2O production in aquaculture ponds. The availability of nitrite is likely a major factor driving partial denitrification for both sedimentary and water column N2O production.

Multiple Ba Phases in Marine Sediments elucidate environmental processes and conditions for the Holocene Mediterranean

Mediterranean sediments are known for their major shifts in sedimentary composition associated with dramatic changes in environmental and climate conditions. A typical expression for this in Holocene sediments is the occurrence of a distinctly organic-rich unit, sapropel S1, thought to be related with humid climate conditions. The S1 is accompanied by major changes in Ba content and in other elemental composition, respectively related to biological upper water column productivity and shifts in detrital sediment provenance. Here, we focus on changes in Ba and in its various phases to unravel shifts in environmental conditions around S1.Using selective extractions for sediments in cores MP37PC, MP39PC and MP50PC, respectively at 1908, 1359 and 775m water depth, we distinguish different Ba-phases: detrital Ba (Ba-det), barite Ba (Ba-bar), Mn-oxide-related Ba (Ba-ox), carbonate- and pyrite-associated Ba (Ba-carb and Ba-pyr). The predominant Ba-phase during S1 is Ba-bar (∼70%), whereas before and after S1 it is Ba-det (>90%). All other phases are mostly relatively small but have diagnostic value for environmental conditions and processes.The detrital (Ba/Al) is distinctly lower during S1 than before and after. The (Ba/Al). det variability, and enhanced values for Ba-carb and Ba-ox may have substantial influence on the traditional way to derive barite-Ba or excess-Ba content.The steep gradients in Ba-bar at onset and ending of S1and its high level during S1, clearly identify this period. In addition, and due to the exclusive nature of extraction results, we unequivocally demonstrate for the first time that there is a small gradual ‘early onset’ Ba-bar preceding the major increase in sapropel S1 formation sensu stricto.The ending of S1 formation is relatively abrupt with a well-defined reventilation event (’6 ka reventilation event’). This reventilation is associated with a rapid change in environmental conditions such as oxygenation and related remineralization of organic matter and recrystallization of carbonate. This rapid change and associated diagenetic processes are authentically recorded in Ba-carb and Ba-ox. For the period during sapropel S1 formation, the Ba-carb and Ba-ox results point to different levels of ventilation in the Eastern Mediterranean, i.e. a relatively mixed water mass down to ∼800m, a moderately mixed water mass from ∼800-to ∼1500 m, and a more stagnant water mass below 1500 m.

Characteristics of Phytoplankton Productivity in Three Typical Lake Zones of Taihu, China

In lake aquatic ecosystems, which form the material and energy base of lakes, primary production is critical. The present study addresses the characteristics of primary phytoplankton productivity and its relationship with environmental physicochemical factors in three typical zones (algae-type, transition, and grass-type) of the eutrophic Lake Taihu. Seasonal sampling was conducted, and black–white bottle oxygen measurement was used to determine the primary productivity in different water layers in the lake. The results show obvious temporal and spatial differences in the physicochemical factors and phytoplankton productivity in Lake Taihu. The water column productivity and respiration conformed to the following seasonal descending order, summer > fall > spring > winter, and the following regional descending order, algae-type zone > transition zone > grass-type zone. The seasonal proportions of primary productivity were approximately 40% in the summer, 25% in the fall, 20% in the spring, and less than 15% in the winter. The highest values of water layer productivity and respiration were mainly at a depth of 0.2 m and decreased with the water depth. The percentage of productivity at different water layers was 23% (0 m), 31% (0.2 m), 23% (0.4 m), 11% (0.6 m), 7% (0.8 m), and 5% (1 m). The optical compensation depth for Lake Taihu was about 0.8 times the transparency (SD). Spearman correlation indicated that the temperature (T) and water depth (D) had an obvious impact on productivity in all three lake zones. Multiple stepwise regression suggested that T, D, SD, and chlorophyll-a (Chl-a) can be used to estimate productivity in different seasons/regions. The main influencing factors on phytoplankton productivity are T, D, Chl-a, and SD in the algae-type and transition zones and T, D, and total suspended solids (TSSs) in the grass-type zone.

Microbial associates of an endemic Mediterranean seagrass enhance the access of the host and the surrounding seawater to inorganic nitrogen under ocean acidification

Seagrasses are important primary producers in oceans worldwide. They live in shallow coastal waters that are experiencing carbon dioxide enrichment and ocean acidification. Posidonia oceanica, an endemic seagrass species that dominates the Mediterranean Sea, achieves high abundances in seawater with relatively low concentrations of dissolved inorganic nitrogen. Here we tested whether microbial metabolisms associated with P. oceanica and surrounding seawater enhance seagrass access to nitrogen. Using stable isotope enrichments of intact seagrass with amino acids, we showed that ammonification by free-living and seagrass-associated microbes produce ammonium that is likely used by seagrass and surrounding particulate organic matter. Metagenomic analysis of the epiphytic biofilm on the blades and rhizomes support the ubiquity of microbial ammonification genes in this system. Further, we leveraged the presence of natural carbon dioxide vents and show that the presence of P. oceanica enhanced the uptake of nitrogen by water column particulate organic matter, increasing carbon fixation by a factor of 8.6–17.4 with the greatest effect at CO2 vent sites. However, microbial ammonification was reduced at lower pH, suggesting that future ocean climate change will compromise this microbial process. Thus, the seagrass holobiont enhances water column productivity, even in the context of ocean acidification.

Unprecedented sea-ice minima enhances algal production deposited at the Arctic seafloor

Sea-ice in the Arctic is declining, with 2018 a particularly low year for ice extent, driven by anomalously warm atmospheric circulation in winter 2017/18. This is consistent with a multi-decadal trend to an earlier ice-free Barents Sea as climate change rapidly warms the Arctic. Here we investigate a N–S transect in the Barents Sea, crossing the Polar Front from Atlantic waters in the south to Arctic waters in the north, focusing on the organic geochemical signature (pigments and lipids) in surface sediments sampled in summer, between the years of 2017–19. Early ice-out in summer 2018 was confirmed by satellite imagery, tracking the evolution of Arctic sea-ice extent between years. Consistent with less extensive sea-ice cover in 2018 we found increases in multiple chlorophyll and carotenoid pigments as well as fatty acids (reflecting recent phytoplankton delivery) in the northern part of our transect at the seafloor. We attribute this to nutrient and organic matter release from earlier 2018 ice-out leading to stratification, post-melt phytoplankton blooms and the deposition of organic matter to the seafloor, evidenced by pigments and lipids. Organic matter delivered to the seafloor in 2018 was reactive and highly labile, confirming its deposition in the most recent season, pointing to rapid deposition. Correlations were found during ice-free periods between satellite-derived chlorophyll a and multiple indicators of water column productivity deposited at the seafloor. We also found convincing evidence of multi-year biogeochemical change across the Polar Front, where sedimentary change is marked by chlorophyll degradation products providing evidence of grazing, indicative of a tightly coupled ecosystem close to the marginal ice zone. Overall, our results show the tight coupling of Arctic productivity with the delivery and quality of organic matter to the seafloor and how this varies across the Barents Sea. More frequent early summer sea-ice loss driven by climate warming in the Barents Sea will have consequences for the delivery of organic matter to the seafloor with impacts for benthic organisms, microbiology and the sequestration of carbon.

Airborne Bioaerosol Observations Imply a Strong Terrestrial Source in the Summertime Arctic

AbstractPrimary biological aerosol (PBA) is a component of coarse mode aerosol which may affect climate and health. The possible climate impacts arise from interactions between PBA and water vapor, especially since some PBA nucleate ice at warm temperatures. The health impacts span from seasonal allergies to transmission of pathogens. Despite their potential importance, the emissions, transport, and atmospheric distribution of PBA are poorly understood, especially at high latitudes where cloud effects could be pronounced. Here we report summertime measurements of fluorescent aerosol (a proxy for PBA) over the Bering and Chukchi Seas using a Wide‐Band Integrated Bioaerosol Sensor aboard a Twin Otter, alongside a lidar which detected water column productivity. Most observations occurred at 300 m over the ocean with periodic excursions to 60 and 900 m. Loadings were always low in the marine boundary layer, despite the presence of subsurface plankton layers and in contrast to other recent reports, likely indicating low oceanic emissions during our study. Large variability was observed in PBA aloft, with higher concentrations approaching those observed over the Continental US. Back trajectory analysis showed that high loadings were associated with recent transit through the continental boundary layer and we estimate PBA emissions from the Arctic tundra of up to 300 m−2 s−1 at the warmest observed temperatures. On days with strong transport from land (∼50% of our flights), PBA accounts for 12% of supermicron number and 64% of supermicron volume, indicating potentially significant effects on the albedo, glaciation and lifetime of Arctic clouds.

Concomitant changes of lipid biomarker and water column mixing since mid-Holocene

An increasing observation of in-situ production of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in marine environments has aroused concerns about the utility of BIT (the ratio of brGDGTs over isoGDGTs) index to indicate soil contribution in marine environment. In this study, GDGT biomarkers and foraminiferal carbon isotope (δ13C) are analyzed in a mid-to-late Holocene sediment core from the northern South China Sea (SCS). The BIT values exhibited an overall similar pattern as compared to Δδ13C(P-G) (the δ13C difference between subsurface and surface planktonic foraminifera) index that indicates water column productivity, implying that the variations in BIT are likely related to marine productivity status. A positive relation between BIT and Δδ13C(P-G) was observed and explained by the downward transport of organic matter due to change of upper water mixing, with higher BIT Δδ13C(P-G) values during strengthened upper water mixing. Despite these findings, further investigations on the mechanistic processes that regulate the productions and dynamics of brGDGTs and isoGDGTs in marine sediments are warranted.

Contribution of photic and aphotic N2 fixation to production in an oligotrophic sea

AbstractDinitrogen (N2) fixation was investigated at a pelagic station in the oligotrophic waters of the northern Gulf of Aqaba, Red Sea, between February 2016 and December 2018. In situ 15N2 and 13C incubations were used to evaluate photic and aphotic N2 fixation rates and diazotrophic contribution to water column productivity. N2 fixation rates were typically low (below detection to <0.5 nmol N L−1 d−1). Maximal rates of 3.1 nmol L−1 d−1, measured at 100 m when conspicuous slicks of the cyanobacterium Trichodesmium appeared on the surface water. Amplicon sequencing of nifH demonstrated that non‐cyanobacterial diazotrophs, mostly α‐ and γ‐proteobacteria comprised the majority (82–100%) of amplicon sequence variants retrieved from photic and aphotic depths when low N2 fixation rates were measured, while amplicons representing cyanobacteria were nearly absent, but appeared in low abundance (~ 2%) when maximal rates were measured. During the stratified summer‐period, water‐column N2 fixation rates (10–75 μmol m−2 d−1) comprised ~ 1–40% of new production (NP). During the winter mixing period N2 fixation rates were considerably higher (11–242 μmol m−2 d−1) but made up only <1% of NP. This is because, during this period, nitrate supplied to the photic zone by the vertical mixing becomes the major N source for NP. We conclude that on an annual average, diazotrophy plays a minor role in the NP of the Gulf. The major “new” nitrogen sources are cross‐thermocline turbulent diffusion of nitrate during summer stratification and vertical mixing during the fall–winter.

Variations of organic matter content and type within the sequence stratigraphic framework of the lacustrine deep-water Dongyuemiao formation, Sichuan Basin, Western China

The organic matter (OM) enrichment patterns and mechanisms in lacustrine deep-water shale successions were controlled by complex geological and biological processes, rather than any single factor. The integration of sedimentology, petrography, geochemistry, and sequence stratigraphy in the study of shale successions is vital to the development of robust paleoenvironmental interpretations and OM enrichment model. Based on a systematic sedimentological, petrographic, and geochemical analysis of three continuous cored wells of the Lower Jurassic Dongyuemiao Formation, Sichuan Basin, within a sequence stratigraphic framework, this study reports the complex geological responses in detrital input, sedimentation rate, benthic redox conditions, primary water column productivity, and OM enrichment pattern that caused by lake-level fluctuation. The results indicate that none of the proxies for detrital input, redox conditions, or productivity exhibit a significant correlation with total organic carbon (TOC) content in sediments. The OM enrichment was regulated by a complex interplay of these three factors, which is a function of relative lake-level fluctuation. In general, detrital input was decreased during lake-level transgression; whereas increased during lake-level regression. Reducing conditions and primary productivity were generally enhanced during lake-level transgression; whereas diminished during lake-level regression. Coupled relationships between lake-level fluctuation, bulk TOC content, and OM types were observed. The bulk TOC content and liptinite proportion in sediments increased gradually during the lake-level transgression. In contrast, the bulk TOC content and liptinite proportion in sediments gradually decreased during the lake-level regression. This general principle applies to both second-order and third-order lake-level fluctuation. Our study demonstrates that the development of a reliable OM enrichment model in lacustrine shale successions requires a comprehensive sequence stratigraphic framework.

The Dominant Role of the Water Column in Nitrogen Removal and N2O Emissions in Large Rivers

AbstractRivers are important sites for nitrogen (N) cycling processes. N removal is thought to occur predominantly in the benthic zone and the role of the water column is frequently neglected. Here, we find that the water‐air and sediment‐water fluxes of N2 and N2O decreased, while the water column production increased with stream order through 4‐year observations across six river networks in China. The water column contribution increased with stream order and accounted for more than 50% in rivers above fifth order. The increase in the contact area of suspended sediment (SPS)‐water caused by higher SPS concentrations and river depths resulted in the shift as river size increased. N removal and N2O emissions would be underestimated approximately by 50% if neglecting water column processes for the six river networks. This study highlights the necessity and provides a pathway to incorporate water column processes into models to balance the global N budget.

Primary productivity measurements in the Ross Sea, Antarctica: a regional synthesis

Abstract. Polar systems are undersampled due to the difficulty of sampling remote and challenging environments; however, these systems are critical components of global biogeochemical cycles. Measurements on primary productivity in specific areas can quantify the input of organic matter to food webs and so are of critical ecological importance as well. However, long-term measurements using the same methodology are available only for a few polar systems. Primary productivity measurements using 14C-uptake incubations from the Ross Sea, Antarctica, are synthesized, along with chlorophyll concentrations at the same depths and locations. A total of 19 independent cruises were completed and 449 stations occupied where measurements of primary productivity (each with seven depths) were completed. The incubations used the same basic simulated in situ methodology for all. Integrated water column productivity for all stations averaged 1.10 ± 1.20 g C m−2 d−1, and the maximum was 13.1 g C m−2 d−1. Annual productivity calculated from the means throughout the growing season equalled 146 g C m−2 yr−1. The mean chlorophyll concentration in the euphotic zone (the 1 % irradiance level) was 2.85 ± 2.68 mg m−3 (maximum observed concentration was 19.1 mg m−3). Maximum photosynthetic rates above the 30 % isolume (normalized to chlorophyll) averaged 0.98 ± 0.71 mg C (mg chl)−1 h−1, similar to the maximum rate found in photosynthesis–irradiance measurements. Productivity measurements are consistent with the temporal patterns of biomass found previously, with biomass and productivity peaking in late December; mixed layers were at a minimum at this time as well. Estimates of plankton composition also suggest that pre-January productivity was largely driven by the haptophyte Phaeocystis antarctica and summer productivity by diatoms. The data set (https://doi.org/10.26008/1912/bco-dmo.863815.2, Smith, 2021) will be useful for a comparison to other Antarctic regions and provide a basis for refined bio-optical models of regional primary productivity and biogeochemical models for the Southern Ocean.

Changes in water column oxygen, estimates of productivity and the development of anoxia in a major embayment of the southern Benguela eastern boundary upwelling system

The metabolic balance of autotrophy and heterotrophy in St Helena Bay was investigated through estimates of net community production (NCP) and respiration (R), as determined by changes in water column oxygen. Compared to adjacent coastal waters St Helena Bay was shown to be an area of low-enrichment and high-retention. Productivity during the upwelling season was found to be nutrient limited in the upper few metres (0–3 m) and light limited through self-shading below these surface waters. During the upwelling season the maintenance of thermal stratification in the bay leads to consistently low nutrients (nitrate x¯ = 1.41 mmol m−3) in the surface waters (0–3 m). High biomass maintained through mechanisms of cyclonic retention and entrainment from the Namaqua shelf underlies the rapid attenuation of light through the water column and the consequent limitation of subsurface productivity. As a result productivity maxima were located in surface waters (0 or 3 m) irrespective of the low nutrient concentrations in these waters and the presence of subsurface biomass maxima. Estimates of gross community production (GCP) normalised to Chl a therefore declined sharply below 3 m depth. The mean community compensation depth (Zcc), where R = GCP, was determined to be 9.16 m and was shallower than the 1% light depth for PAR (Z1%PAR) at 15.42 m. The role of light in limiting productivity was clearly evident in that 48% of water column biomass was located below the Zcc. The importance of the light environment in controlling the seasonality of productivity was further evident in that daily rates of productivity in surface waters normalised to Chl a were clearly linked to day length. Phytoplankton community composition and successional status were also shown to influence productivity with the contribution of R to GCP increasing with the succession of phytoplankton communities. Estimates of mean water column Chl a (8.60 mg m−3 as determined from discrete water samples and 8.40 mg m−3 as estimated from profiles of fluorescence) and mean integrated water column productivity (5.38 g C m−2 d−1 as determined by the light-and-dark bottle oxygen method) were ~ 1.5 fold higher than past measurements most of which were made during the late 1970s and 80s. Oxygen depletion within St Helena Bay is locally enhanced as a result of the entrainment and retention of biomass in St Helena Bay and causes it to function as an area of higher carbon sequestration.

Biological production in two contrasted regions of the Mediterranean Sea during the oligotrophic period: an estimate based on the diel cycle of optical properties measured by BioGeoChemical-Argo profiling floats

Abstract. This study assesses marine community production based on the diel variability of bio-optical properties monitored by two BioGeoChemical-Argo (BGC-Argo) floats. Experiments were conducted in two distinct Mediterranean systems, the northwestern Ligurian Sea and the central Ionian Sea, during summer months. We derived particulate organic carbon (POC) stock and gross community production integrated within the surface, euphotic and subsurface chlorophyll maximum (SCM) layers, using an existing approach applied to diel cycle measurements of the particulate beam attenuation (cp) and backscattering (bbp) coefficients. The diel cycle of cp provided a robust proxy for quantifying biological production in both systems; that of bbp was comparatively less robust. Derived primary production estimates vary by a factor of 2 depending upon the choice of the bio-optical relationship that converts the measured optical coefficient to POC, which is thus a critical step to constrain. Our results indicate a substantial contribution to the water column production of the SCM layer (16 %–42 %), which varies largely with the considered system. In the Ligurian Sea, the SCM is a seasonal feature that behaves as a subsurface biomass maximum (SBM) with the ability to respond to episodic abiotic forcing by increasing production. In contrast, in the Ionian Sea, the SCM is permanent, primarily induced by phytoplankton photoacclimation, and contributes moderately to water column production. These results clearly demonstrate the strong potential for transmissometers deployed on BGC-Argo profiling floats to quantify non-intrusively in situ biological production of organic carbon in the water column of stratified oligotrophic systems with recurring or permanent SCMs, which are widespread features in the global ocean.

Chemostratigraphy and Paleoenvironment of the Miocene Organic Rich Sediments in the East Kutai Sub-Basin, Indonesia

The Miocene sedimentary rocks in Samarinda area constrains organic rich sediments, which are considered as a good source rocks hydrocarbon in the East Kutai Sub-Basin, Kalimantan. The high organic material content within the sediments is related to the dynamics of depositional environment in deltaic setting. The accumulation and characteristics of organic matter in this area may be influenced by multiple factors, under a complex physical-chemical processes. Geochemical data of major and trace elements obtained for a total 309 outcrop samples from four locations were interpreted to define chemostratigraphic and paleoenvironmental conditions (paleoproductivity, detrital influx, paleoredox and paleosalinity) responsible for organic carbon accumulation and source rocks characterization. Stratigraphic variation in inorganic geochemistry allows two chemostratigraphic packages to be defined and correlated within the Miocene sedimentary sequences. These chemostratigraphic packages are geochemically differentiated using SiO2/Al2O3, TiO2/Al2O3, Na2O/Al2O3, TiO2/Nb and Sr/Ba ratio values. The chemical alteration index (CIA) suggests that the sedimentary unit was deposited in a hot and humid climate, with moderate to intensive weathering intensity. Detrital material input proxies (Si/Al, Ti/Al) indicate that the low Si/Al and Ti/Al ratios reflect a low material input providing an increasing organic matter accumulation in the Middle Miocene. However, paleoproductivity proxies (P/Ti, Ba/Al) show the organic matter enrichment is not restrained by water column productivity, as indicated by a weak correlation between TOC and productivity index. In addition, paleosalinity index (Sr/Ba) and redox indicators (V/Cr, V/Sc U/Th and Mo/Al) indicate that the sediments were deposited in a brackish environment with dysoxic to suboxic conditions and might be the main control in the enrichment of organic matter in the study area. Thus, the detrital material influx and paleoredox conditions controlled organic accumulation and source characteristics the Miocene sedimentary sequence of the Kutai Basin.Keywords: Chemostratigraphy, Kutai Basin, paleoenvironment, source rocks.

Tight benthic-pelagic coupling drives seasonal and interannual changes in iron‑sulfur cycling in Arctic fjord sediments (Kongsfjorden, Svalbard)

Glaciated fjords are dynamic systems dominated by seasonal events such as spring phytoplankton blooms and pulses of glacial sediment-bearing meltwater delivery. These fjords are also characterized by strong spatial gradients in environmental factors such as sedimentation rate and primary productivity from the glacier-influenced head to the marine-influenced mouth. Such seasonal variations and spatial gradients, combined with the ongoing influence of climate change, generate non-steady state conditions, which have a strong impact on the mineralization of organic carbon in the fjord sediments and the flux of nutrients from the seabed. In order to investigate the role of fjord seasonal events and variability on diagenetic cycling of iron (Fe) and sulfur (S), we sampled Kongsfjorden (Svalbard, 79°N) in the spring, mid-summer, and late summer. We investigated sediment structure and biogeochemistry, conducted laboratory experiments to determine reaction rates, and compared these findings to water column productivity and turbidity. We found that rapid sedimentation near the glacial input buried algal matter-rich layers that fueled sub-surface peaks in mineralization rates over multi-year timescales. Sulfate reduction rates were limited by organic carbon availability and competition with Fe-reducers, while Fe reduction was controlled by the availability of reactive Fe(III) oxides. Pore water Fe 2+ concentrations were influenced by sulfur cycling pathways and abiotic reactions such as carbonate precipitation and potentially reverse weathering. Seasonal changes in sedimentation and organic carbon supply caused lower sulfate reduction and sulfide production rates in spring, driving generally higher spring fluxes of Fe 2+ from the sediment. The results of this study reveal the potential for an increased benthic source of nutrients such as Fe with continued benthic remineralization over winter in Kongsfjorden. Interannual changes in primary productivity, which are likely to intensify with global warming, and shifts in glacial sediment delivery have immediate impacts on the benthic cycling of Fe and S in this tightly coupled system, with a long term trend likely toward decreased benthic Fe fluxes. With the glacial retreat and changes in productivity predicted due to climate change, glaciated fjords such as Kongsfjorden may become a less efficient carbon sink by burying less terrestrial and marine-sourced organic matter in the deep sediments. • Investigation of seasonal Fe-S-C cycling in Arctic fjord sediments and water column. • Results show benthic respiration and increased benthic Fe 2+ flux over winter. • Findings suggest that fjord sediments respond rapidly to water column changes. • With glacial retreat, fjords may produce less benthic Fe and sequester less carbon.

Ecological Constraints of Plankton Bio-Indicators for Water Column Stratification and Productivity: A Case Study of the Holocene North Aegean Sedimentary Record

This study presents novel findings on the drivers of the calcitic planktonic foraminiferal and aragonitic pteropod Holocene assemblages of the North Aegean Trough (northeastern Mediterranean), an area recording the interaction between dynamic water masses as they exchange between the northern and southern Mediterranean sub-basins. Both of these groups of microorganisms are the major producers of calcium carbonate in the ocean, and are particularly sensitive to climate and oceanographic changes over the late Quaternary. Downcore micropaleontological data from the gravity core AEX-15, supplemented with multivariate statistical Q-mode cluster and principal component analyses (PCA) results, provide significant insights on the water column dynamics during the Holocene. Focusing on the last ~10 calibrated thousands of years before the present day (ka cal BP), our integrated study reveals that primary productivity is the dominant factor controlling the planktonic foraminifera distribution in the North Aegean Sea, whereas water column stratification, and particularly the intensity of the oxygen minimum zone, seems to be the major driver on the pteropod distribution. Besides productivity and thermal stratification, which show the highest explanatory power for planktonic foraminifera and pteropod communities, respectively, though they affect both groups to a different extent, upwelling seems to further affect both faunal groups. Overall, our findings are consistent with those derived by published late Quaternary eastern Mediterranean records, highlighting in parallel a useful additional dimension on planktonic foraminiferal and pteropod ecology, which is inextricably linked with the factors of primary productivity and vertical stratification of the warm Holocene water column.

Organic Geochemical Characteristics of Mudstone and Marl from Western Hoh Xil Basin of Tibet

The mudstone and marl from western Hoh Xil basin, located in Tibet of the west of China, were deposited in Tertiary lacustrine environment. Investigation of organic geochemistry, sedimentary characteristics, and 13C in kerogen was conducted to analyze the sedimentary environment, biomarkers, paleoclimate, and source of organic matter during deposition. The Cenozoic sedimentary facies of the basin included upper lacustrine facies and lower alluvial fan facies, which belong to Miocene Wudaoliang Formation and Oligocene Yaxicuo Group, respectively. The Miocene marl-sandstone-mudstone from Wudaoliang Formation was analyzed. Maceral composition was dominated by amorphous organic matter. T max values indicated that the mudstones were thermally immature-low maturity with mainly type II and III organic matter, while organic matter in marlite belongs mainly to type I-II1 with low maturity-maturity stage. The biomarkers showed the characteristics of odd-over-even predominance of long-chain n-alkanes, higher proportion of C27 sterane in most of the samples, heavy δ13Corg composition, low Pr/Ph ratios (0.11-0.36), and so on. Organic geochemistry indicated that the organic matter originated from bacteria, algae, and higher plants. The rocks were formed in reducing environments with stratified water column and high productivity. The paleoclimate became more humid during depositional stage in the western Hoh Xil basin.

An integrated study of the petrographic and geochemical characteristics of organic-rich deposits of the Wufeng and Longmaxi formations, western Hubei Province, South China: Insights into the co-evolution of paleoenvironment and organic matter accumulation

Enhanced productivity and/or widespread anoxia have been postulated as the dominant controls on organic matter (OM) accumulation in South China during the Ordovician-Silurian (O-S) transition. However, their roles appear to have varied in space and time, and causal mechanisms linking OM accumulation with changing paleoenvironmental conditions remain poorly constrained. Here we report on results of a multi-faceted investigation of the Wufeng and Longmaxi formations in a drilled core from Xianfeng County, western Hubei Province, South China. The roles played by paleoredox conditions, paleoproductivity, paleoclimate, terrigenous input, upwelling, and volcanism on OM accumulation and preservation during the O-S transition are considered. Our results suggest that the depositional history of OM accumulation as recorded by the studied succession can be subdivided into five phases, each one represented by a lithostratigraphic unit. Unit 1 (average TOC = 3.28%) comprises siliceous shale deposited under generally suboxic conditions interrupted by brief episodes of anoxia beneath very productive surface water during the late Katian time (~447.62 Ma). Seasonal upwelling-driven productivity appears to have exerted fist-order control on OM accumulation. Overlying siliceous organic-rich shale of unit 2 (average TOC = 4.93%) records anoxic bottom water conditions from the latest Katian to Hirnantian time. Enhanced productivity and OM preservation may have been maintained by phosphorous cycling. Calcareous shale of the overlying Guanyingqiao unit 3 (TOC = 2.8%) accumulated under suboxic conditions and low to moderate water column productivity during the late Hirnantian Stage. Climate-driven sea level fall was responsible for diminished OM accumulation at this time. Siliceous clay-rich shale of unit 4 (average TOC = 5.53%) was deposited beneath highly productive surface water under anoxic bottom water conditions at the beginning of the Rhuddanian. The presence of volcanic ash and enriched Hg concentrations in these deposits suggests that productivity was enhanced by volcanism. Overlying silty shale of unit 5 (average TOC = 1.71%) was deposited in the middle to upper Rhuddanian under dominantly suboxic conditions, declining productivity levels, and increased delivery of terrigenous sediments. Accumulation of these deposits was contemporaneous with a period of falling sea level related to the Kwangsian Orogeny that would have curbed OM accumulation. Our results, considered in tandem with those of studies from shallow to deep water regions of the Yangtze Block, reveal a level of spatial heterogeneity of redox conditions and productivity levels across the Yangtze Sea during the O-S transition manifested by regional differences in OM accumulation mechanisms and histories.

Offshore Wind Farm Footprint on Organic and Mineral Particle Flux to the Bottom

Offshore wind farms (OWFs) are an important source of renewable energy accounting for 2.3% of the European Union's electricity demand. Yet their impact on the environment needs to be assessed. Here, we couple a hydrodynamic (including tides and waves) and sediment transport model with a description of the organic carbon and mineral particle dynamics in the water column and sediments. The model is applied to the Belgian Coastal Zone (BCZ) where OWFs currently occupy 7% of its surface area which is estimated to double in the next 5 years. The impact of OWFs on the environment is represented through the filtration of the water column and fecal pellets production by the blue mussel, the dominant fouling organism. Our model simulations show that the impact of biodeposition on the mud particle sedimentation and on sediment composition is small compared to the fluxes associated with tidal deposition and resuspension and the lateral inputs. In contrast, the total organic carbon (TOC) flux to the sediment is significantly altered inside the OWF perimeters and TOC deposition is increased up to 50% in an area 5 km around the monopiles. Further away, the TOC flux to the bottom decreases with a notable effect up to 30 km away. The major changes are found along the direction of the main residual current and tidal ellipse's major axis. In addition, sub-mesoscale gyres act as retention areas with increased carbon deposition. A future OWF in the BCZ will be located close to gravel beds in a Natura 2000 area, considered as vulnerable habitats and biodiversity hotspots. The different scenarios for this OWF, varying in turbine number and positioning, are compared in terms of impact on the carbon and mineral particle deposition flux in the BCZ and, particularly, to these gravel beds. The scenarios show that the number of turbines has only a slight impact on the TOC deposition flux, unlike their positioning that significantly alters the TOC flux to the gravel beds. The TOC deposition flux exceeds 50%, when the turbines are placed next to the gravel beds; while a limited increase is simulated, when the turbines are located the farthest possible from them.