Stratified Water Column Research Articles

Messinian bottom-grown selenitic gypsum: An archive of microbial life.

Primary gypsum deposits, which accumulated in the Mediterranean Basin during the so-called Messinian salinity crisis (5.97-5.33Ma), represent an excellent archive of microbial life. We investigated the molecular fossil inventory and the corresponding compound-specific δ13 C values of bottom-grown gypsum formed during the first stage of the crisis in four marginal basins across the Mediterranean (Nijar, Spain; Vena del Gesso, Italy; Heraklion, Crete; and Psematismenos, Cyprus). All studied gypsum samples contain intricate networks of filamentous microfossils, whose phylogenetic affiliation has been debated for a long time. Petrographic analysis, molecular fossil inventories (hydrocarbons, alcohols, and carboxylic acids), and carbon stable isotope patterns suggest that the mazes of filamentous fossils represent benthic microbial assemblages dominated by chemotrophic sulfide-oxidizing bacteria; in some of the samples, the body fossils are accompanied by lipids produced by sulfate-reducing bacteria. Abundant isoprenoid alcohols including diphytanyl glycerol diethers (DGDs) and glycerol dibiphytanyl glycerol tetraethers (GDGTs), typified by highly variable carbon stable isotope composition with δ13 C values spanning from -40 to -14‰, reveal the presence of planktic and benthic archaeal communities dwelling in Messinian paleoenvironments. The compound inventory of archaeal lipids indicates the existence of a stratified water column, with a normal marine to diluted upper water column and more saline deeper waters. This study documents the lipid biomarker inventory of microbial life preserved in ancient gypsum deposits, helping to reconstruct the widely debated conditions under which Messinian gypsum formed.

Sea-ice, primary productivity and ocean temperatures at the Antarctic marginal zone during late Pleistocene

While Pleistocene glacial-interglacial cycles are commonly associated with strong waxing and waning of Northern Hemisphere ice sheets, the response of the Antarctic ice sheet and regional changes in oceanographic and environmental conditions to Pleistocene climate dynamics remain poorly constrained. We present a reconstruction of sea-ice cover, sea surface temperature and primary productivity off the Ross Sea margin (Adare Basin at the slope of the Drygalski Basin) during the marine isotope stages (MIS) 9 to 5 (350–70 thousands years ago, encompassing Terminations IV to II). Our multiproxy study relies on micropaleontology (diatoms, dinoflagellate cysts, benthic foraminifers), organic and inorganic geochemistry proxies (carbon and nitrogen isotopes, lipid biomarkers, XRF-data), and sedimentology (IRD) obtained from deep-sea core AS05-10. For each glacial-interglacial transition a clear succession of events can be observed: (near-)permanent sea ice cover during glacial stages is followed by ice-shelf break-up with episodic ice-free areas and surface water stratification. Notably, ice-shelf break-up precedes the increase in air temperature as measured in the Vostok ice core for each glacial-interglacial transition. Generally, air temperature over Vostok starts rising once sea-ice cover at site AS05-10 has significantly decreased, becoming seasonal, as indicated by the diatom species composition. This is also reflected by the high diatom productivity and increased water mixing at site AS05-10, which is indicative of its proximity to the Marginal Ice Zone. At the onset of Termination II (MIS6 to 5), high export productivity and dysoxic bottom water conditions occurred, while water temperature increased about 5 °C. During each interglacial spring/summer sea-ice cover is most reduced, and highest productivity occurs. Following each interglacial, the warm and cold fluctuations match the sawtooth character of the temperatures over Vostok. This record illustrates that at the Ross Sea margin, sea surface conditions and (export) productivity were strongly influenced by the natural climate variability of the Pleistocene. In light of this, current global warming may lead to increased ice-shelf break-up, water column stratification and shifts in the position/size of the Marginal Ice Zone with implications for algal species composition and diversity, and for primary productivity.

Orbital tuning for the middle Eocene to early Oligocene Monte Cagnero Section (Central Italy): Paleoenvironmental and paleoclimatic implications

During the middle Eocene to early Oligocene Earth transitioned from a greenhouse to an icehouse climate state. The interval comprises the Middle Eocene Climatic Optimum (MECO; ~40 Ma) and a subsequent long-term cooling trend that culminated in the Eocene-Oligocene transition (EOT; ~34 Ma) with the Oi-1 glaciation. Here, we present a refined calcareous nannofossil biostratigraphy and an orbitally tuned age model for the Monte Cagnero (MCA) section spanning the middle Eocene to the early Oligocene (~41 to ~33 Ma). Spectral analysis of magnetic susceptibility (MS) data displays strong cyclicities in the orbital frequency band allowing us to tune the identified 405 kyr eccentricity minima in the MS record to their equivalents in the astronomical solution. Our orbitally tuned age model allows us to estimate the position and duration of polarity chrons (C18 to C13) and compare them with other standard and orbitally tuned ages. We were also able to constrain the timing and duration of the MECO event, which coincides with a minimum in the 2.4 Myr and 405 kyr eccentricity cycles. Our study corroborates the previous estimated age for the base of the Rupelian stage (33.9 Ma) and estimates the base of the Priabonian stage in the MCA section to be at 37.4 Ma. Finally, calcareous nannofossils with known paleoenvironmental preferences suggest a gradual shift from oligotrophic to meso-eutrophic conditions with an abrupt change at ~36.8 Ma. Besides, nannofossil assemblages suggest that enhanced nutrient availability preceded water cooling at the late Eocene. Altogether, this evidence points to a poorly developed water column stratification prior to the cooling trend.

The consequences of chaos: Foraging activity of a marine predator remains impacted several days after the end of a storm.

As extreme weather is expected to become more frequent with global climate change, it is crucial to evaluate the capacity of species to respond to short-term and unpredictable events. Here, we examined the effect of a strong storm event during the chick-rearing stage of little penguins (Eudyptula minor) from a mega colony in southern Australia. We investigated how a 3-day storm affected the foraging behaviour of little penguins by comparing their foraging activities and body mass change before, during and after the storm event. As strong winds deepened the mixed layer in the birds’ foraging zone during the storm, penguins increased their foraging trip duration, had a lower prey encounter rate and a lower body mass gain. The adverse effects on the foraging efficiency of little penguins continued several days after the storm ceased; even though the water column stratification had returned as before the storm, suggesting a prolonged effect of the storm event on the prey availability. Thus, short-term stochastic events can have an extended impact on the foraging efficiency of penguins. When occurring at a crucial stage of breeding, this may affect breeding success.

Upper ocean variability off NE Greenland (79°N) since the last glacial maximum reconstructed from stable isotopes in planktic foraminifer morphotypes

We report on stable oxygen and carbon isotope data obtained on two different morphotypes of polar planktic foraminifers, i.e., fully encrusted and minor encrusted Neogloboquadrina pachyderma, from a sediment core taken on the NE Greenland continental margin. These morphotypes are supposed to live at different water depths of the upper water column in the area which today is strongly stratified, with a low-saline, cold-water layer at the surface. The paired isotopic data sets inform on temporal variations of past water salinity and temperature in the preferred water depth ranges of the investigated morphotypes and allow conclusions on the stratification of the upper water column. The radiocarbon-dated sediment core covers the time interval from 21 to 4 cal-ka, but the early part of the deglacial interval (18.5–12.7 cal-ka) is not represented, probably due to intense erosion by bottom currents. In sediments from the late last glacial maximum, oxygen isotope differences between thin-shelled and thick-shelled N. pachyderma are low and point at a weaker stratification with less freshwater than today near the surface. The carbon isotopes indicate a dense, perennial sea ice cover, very limited bioproduction, and the presence of a subsurface Atlantic Water layer. In the late deglaciation until ∼10.3 cal-ka, the stable isotope values of both analyzed morphotypes are considerably lower, with significantly stronger amplitudes in the record of thin-shelled specimens than later on. The high-amplitude record stems from a laminated sediment sequence whose older part was deposited within just a few decades. The data are evidence of a strong freshwater event in the research area that probably started before 12.7 ka and may have reduced sea surface salinities by 4–5 practical salinity units. As freshwater sources we discuss both the disintegration of NE Greenland shelf ice and export from the Arctic Ocean interior. The event may have contributed to the weakening of the Atlantic meridional overturning circulation during the Younger Dryas cold event. For the early and mid-Holocene (10-4 cal-ka), the isotope data suggest a structure of the upper water column similar to today, with a well-developed halocline separating low-saline near-surface waters from the underlying Atlantic Water layer. A seasonally disintegrated sea ice cover allowed for a considerable planktic bioproduction.

Hydrostatic Densitometer for Monitoring Density in Freshwater to Hypersaline Water Bodies

Density, temperature, salinity, and hydraulic head are physical scalars governing the dynamics of aquatic systems. In coastal aquifers, lakes, and oceans, salinity is measured with conductivity sensors, temperature is measured with thermistors, and density is calculated. However, in hypersaline brines, the salinity (and density) cannot be determined by conductivity measurements due to its high ionic strength. Here, we resolve density measurements using a hydrostatic densitometer as a function of an array of pressure sensors and hydrostatic relations. This system was tested in the laboratory and was applied in the Dead Sea and adjacent aquifer. In the field, we measured temporal variations of vertical profiles of density and temperature in two cases, where water density varied vertically from 1.0 × 103 kg·m−3 to 1.24 × 103 kg·m−3: (i) a borehole in the coastal aquifer, and (ii) an offshore buoy in a region with a diluted plume. The density profile in the borehole evolved with time, responding to the lowering of groundwater and lake levels; that in the lake demonstrated the dynamics of water-column stratification under the influence of freshwater discharge and atmospheric forcing. This method allowed, for the first time, continuous monitoring of density profiles in hypersaline bodies, and it captured the dynamics of density and temperature stratification.

Microbial metagenome-assembled genomes of the Fram Strait from short and long read sequencing platforms.

The impacts of climate change on the Arctic Ocean are manifesting throughout the ecosystem at an unprecedented rate. Of global importance are the impacts on heat and freshwater exchange between the Arctic and North Atlantic Oceans. An expanding Atlantic influence in the Arctic has accelerated sea-ice decline, weakened water column stability and supported the northward shift of temperate species. The only deep-water gateway connecting the Arctic and North Atlantic and thus, fundamental for these exchange processes is the Fram Strait. Previous research in this region is extensive, however, data on the ecology of microbial communities is limited, reflecting the wider bias towards temperate and tropical latitudes. Therefore, we present 14 metagenomes, 11 short-read from Illumina and three long-read from PacBio Sequel II, of the 0.2–3 µm fraction to help alleviate such biases and support future analyses on changing ecological patterns. Additionally, we provide 136 species-representative, manually refined metagenome-assembled genomes which can be used for comparative genomics analyses and addressing questions regarding functionality or distribution of taxa.

Arid climate disturbance and the development of salinized lacustrine oil shale in the Middle Jurassic Dameigou Formation, Qaidam Basin, northwestern China

The lacustrine Dameigou organic-rich shale of the northern Qaidam Basin is a particularly promising play for recent unconventional oil and gas exploration in China. Deposition was associated with arid intervals during the otherwise humid Middle Jurassic, but the depositional mechanism and organic matter enrichment processes are poorly understood. This study integrates high-resolution organic and inorganic geochemical proxies with detailed sedimentary observations for samples from the Chaiye (CY1) borehole, located in the Yuqia Depression, to investigate the depositional environment and development of the Dameigou shales. Four major lithofacies, including sandy mudstones, oil shale, organic-rich mudstone, and argillaceous mudstone, were identified through detailed microscopic observation and mineralogical analyses. Geochemical paleoclimate proxies (CIA, Fe/Mn and Mg/Ca) indicate that humidity fluctuations primarily drove the lithofacies variation. Other geochemical proxies, including terrigenous supply (Al2O3, Ti2O and regular sterane), lake salinity (Sr/Ba, S/TOC and Ga), redox conditions (V/Cr, U/Th and Pr/Ph), and primary productivity (Baxs, Pxs) indicate that climatic variability exerted a first-order control on the chemical evolution of the lake and the development of organic-rich shale in the Yuqia area. During arid intervals, fresh water supply through precipitation and river runoff weakened, leading to an increase in salinity and water column stratification. As a result, surface water became brackish and oxygen-rich, promoting growth of algae, but density stratification drove the bottom waters to anoxia, providing favorable conditions for organic matter preservation. Recycling of nutrients such as phosphorus under anoxic conditions further promoted eutrophication and high productivity in the surface water, which ultimately promoted the precipitation of carbonate minerals.

Cyclone Hudhud-eddy induced phytoplankton bloom in the northern Bay of Bengal using a coupled model

The present study explored the role of the pre-existing cyclonic eddy on the chlorophyll-a (Chla) bloom after a cyclone pass over the northern Bay of Bengal (BoB). We used a high-resolution (~9 km) Regional Ocean Modeling System (ROMS) coupled with an established Nutrient-Phytoplankton-Zooplankton-Detritus (NPZD) model. In the study, we have considered cyclone Hudhud (category-4), which passed over a cyclonic eddy in the BoB during 7–12 October 2014. Before the passage of cyclone Hudhud, strong stratification in the upper ocean inhibited eddy-driven upwelling of nutrients to the mixed layer and limited the phytoplankton production. However, when cyclone Hudhud interacted with the cyclonic eddy, which further reduced the water column stability (one order less) of the upper ocean and initiated the wind-driven upwelling. This mechanism resulted in the easy upwelling of cold nutrient-rich water to the mixed layer. A case study presented here demonstrates ocean–atmosphere (eddy-cyclone) interaction and its influence on the mixed layer Chla production in the northern BoB. Our study showed that column integrated Chla value in the upper 100 m increased by 70% when cyclone Hudhud passed over a cyclonic eddy region in the BoB. The present study suggested that cyclonic eddy accompanied by cyclone Hudhud induced the Chla bloom in the Northern Bay during the post-monsoon season of 2014.

The impact of the freeze–melt cycle of land-fast ice on the distribution of dissolved organic matter in the Laptev and East Siberian seas (Siberian Arctic)

Abstract. Permafrost degradation in the catchment of major Siberian rivers, combined with higher precipitation in a warming climate, could increase the flux of terrestrially derived dissolved organic matter (tDOM) into the Arctic Ocean (AO). Each year, ∼ 7.9 Tg of dissolved organic carbon (DOC) is discharged into the AO via the three largest rivers that flow into the Laptev Sea (LS) and East Siberian Sea (ESS). A significant proportion of this tDOM-rich river water undergoes at least one freeze–melt cycle in the land-fast ice that forms along the coast of the Laptev and East Siberian seas in winter. To better understand how growth and melting of land-fast ice affect dissolved organic matter (DOM) dynamics in the LS and ESS, we determined DOC concentrations and the optical properties of coloured dissolved organic matter (CDOM) in sea ice, river water and seawater. The data set, covering different seasons over a 9-year period (2010–2019), was complemented by oceanographic measurements (T, S) and determination of the oxygen isotope composition of the seawater. Although removal of tDOM cannot be ruled out, our study suggests that conservative mixing of high-tDOM river water and sea-ice meltwater with low-tDOM seawater is the major factor controlling the surface distribution of tDOM in the LS and ESS. A case study based on data from winter 2012 and spring 2014 reveals that the mixing of about 273 km3 of low-tDOM land-fast-ice meltwater (containing ∼ 0.3 Tg DOC) with more than 200 km3 of high-tDOM Lena River water discharged during the spring freshet (∼ 2.8 Tg DOC yr−1) plays a dominant role in this respect. The mixing of the two low-salinity surface water masses is possible because the meltwater and the river water of the spring freshet flow into the southeastern LS at the same time every year (May–July). In addition, budget calculations indicate that in the course of the growth of land-fast ice in the southeastern LS, ∼ 1.2 Tg DOC yr−1 (± 0.54 Tg) can be expelled from the growing ice in winter, together with brines. These DOC-rich brines can then be transported across the shelves into the Arctic halocline and the Transpolar Drift Current flowing from the Siberian Shelf towards Greenland. The study of dissolved organic matter dynamics in the AO is important not only to decipher the Arctic carbon cycle but also because CDOM regulates physical processes such as radiative forcing in the upper ocean, which has important effects on sea surface temperature, water column stratification, biological productivity and UV penetration.

Organic matter provenance and accumulation of transitional facies coal and mudstone in Yangquan, China: Insights from petrology and geochemistry

The coal measure strata of the late Paleozoic Taiyuan Formation in northern China, which are mainly composed of coal, mudstone, limestone, and sandstone, with obvious cyclothems, are important with respect to the development of unconventional natural gas. However, the organic matter provenance and enrichment mechanism in transitional facies are still controversial. In this study, the organic geochemistry, elemental geochemistry, and petrology of coal and mudstone in Yangquan were investigated. The Pr/nC17 values of n-alkanes as well as the proximate analysis indicate a high level of organic matter maturity. Organic petrology and biomarker compounds show that terrigenous higher plants represent the primary source of the organic matter in coal, with mudstone having larger aquatic organism contribution in lagoon facies. The redox index represents the larger Ceanom, U/Th, and δU values in lagoon facies mudstone, indicating weakly reducibility. Large diameter regular hexagon-shaped pyrite particles were identified in coal and coaly mudstone, indicating oxygen-containing bottom water condition. The rapid deposition rate promotes the accumulation of organic matter in coal and coaly mudstone, and the preservation of organic matter in coal is related to the redox shock. When the climate became warm and dry gradually, the input of terrestrial debris increased, and coal deposits were interrupted. The sudden transgression promotes the stratification of water column, forming oxygen-deficient environment and the growth of algae. The shallow water sediments were covered with gray-black mudstone, and the enrichment of organic matter was controlled by redox conditions.

Multiproxy analysis of paleoenvironmental, paleoclimatic and paleoceanographic changes during the early Danian in the Caravaca section (Spain)

After the Chicxulub impact and mass extinction at the Cretaceous-Paleogene boundary (K-PgB), ecosystems haltingly recovered under unstable conditions. An early Danian (65.9 Ma) perturbation of the carbon cycle known as Dan-C2, which includes two carbon isotopic excursions (CIEs), has been ascribed to inputs of greenhouse gases through large-scale volcanism of the Deccan Traps. However, the relationship between Dan-C2, volcanism and environmental and climatic changes during the early Danian remains ambiguous. Based on stable isotopes, calcium carbonate content, magnetic susceptibility and planktic foraminifera, we present a paleoenvironmental, paleoclimatic and paleoceanographic reconstruction of the early Danian from the Caravaca section, Spain, one of the most complete and continuous K-PgB sections worldwide. The paleobiological response of planktic foraminifera suggests very volatile environmental conditions during the first 230 kyr of the Danian, as reflected in the rapid succession of opportunistic/generalist blooms and episodic high occurrences of aberrant specimens. According to our age model, the Dan-C2 has been identified at the Caravaca section from 65.92 to 65.74 Ma. No evidence of strong carbonate dissolution through ocean acidification was observed in the Dan-C2 interval or the rest of the studied section, excluding the K-PgB clay bed. We find that blooms of highly eutrophic Chiloguembelitria and increases in aberrant planktic foraminifera coincided with a major early Danian eruptive episode of Deccan Traps (Ambelani Formation), occurring before the Dan-C2. Conversely, during both Dan-C2 CIEs, less opportunistic taxa thrived, indicating changes in the upper part of the water column. This study demonstrates that the relationship between marine biota and climate change was very complex and rapidly changing during the early Danian. In addition, we propose that the Deccan volcanism had adverse effects on marine plankton, mostly through strong eutrophication, while an increased water column stratification during the Dan-C2 event resulted in a transient boost in the recovery of ecosystems.

Niche partitioning of bacterial communities along the stratified water column in the Black Sea.

The Black Sea is the largest semi‐closed permanently anoxic basin on our planet with long‐term stratification. The study aimed at describing the Black Sea microbial community taxonomic and functional composition within the range of depths spanning across oxic/anoxic interface, and to uncover the factors behind both their vertical and regional differentiation. 16S rRNA gene MiSeq sequencing was applied to get the data on microbial community taxonomy, and the PICRUSt pipeline was used to infer their functional profile. The normoxic zone was mainly inhabited by primary producers and heterotrophic prokaryotes (e.g., Flavobacteriaceae, Rhodobacteraceae, Synechococcaceae) whereas the euxinic zone—by heterotrophic and chemoautotrophic taxa (e.g., MSBL2, Piscirickettsiaceae, and Desulfarculaceae). Assimilatory sulfate reduction and oxygenic photosynthesis were prevailing within the normoxic zone, while the role of nitrification, dissimilatory sulfate reduction, and anoxygenic photosynthesis increased in the oxygen‐depleted water column part. Regional differentiation of microbial communities between the Ukrainian shelf and offshore zone was detected as well, yet it was significantly less pronounced than the vertical one. It is suggested that regional differentiation within a well‐oxygenated zone is driven by the difference in phytoplankton communities providing various substrates for the prokaryotes, whereas redox stratification is the main driving force behind microbial community vertical structure.

The Stratified Distribution of Dissolved Organic Matter in an AMD Lake Revealed by Multi-sample Evaluation Procedure.

As a typical extreme environment, acid mine drainage (AMD) has been extensively studied for its biogeochemical cycle, but little is known about the quality of dissolved organic matter (DOM) in AMD. In this study, DOM molecules in an AMD lake were detected with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and the change of DOM molecules in the stratified water column was analyzed with a multi-sample evaluation procedure. The results demonstrate that DOM quality is highly stratified and can be linked with severe biogeochemical gradients. In the surface layer, DOM molecules can be distinguished by low quantities and intensities, as well as potential photodegradation products. Oxygen-poor and oxygen-rich molecules alternately dominate the chemocline, which can be explained by the redox-dependent adsorption/desorption of DOM on metastable secondary minerals. A rich and abundant DOM pool with a high proportion of heteroatoms exists at the bottom which can be significantly influenced by material exchange with sediments. These findings emphasize the active role of DOM in extreme AMD environments and expand the understanding of the carbon cycle in the hydrosphere.

Benthic oxygen dynamics and implication for the maintenance of chronic hypoxia and ecosystem degradation in the Berre lagoon (France)

Chronic hypoxia and anoxia have strong impacts on coastal ecosystems worldwide. In shallow coastal ecosystems, such situations are essentially driven by high benthic oxygen (O2) demand resulting from organic matter mineralization in surface sediment and amplified by a low mixing of the water column. However, the benthic O2 demand may greatly vary according to the O2 availability, sediment biogeochemical properties, and bioturbation by macrobenthic fauna. Here we examined how the sediment O2 demand varies in response to seasonal and long-lasting (pluri-decadal) hypoxia in the Berre lagoon, a coastal ecosystem impacted by chronic hypoxia events since 60 years. Oxygen penetration depth, diffusive and total O2 fluxes were measured in situ using a microelectrode autonomous profiler and benthic chamber deployments at three sites impacted by quite-permanent (PA), seasonal (PI) and occasional (PO) hypoxia in August 2016. They were seasonally repeated at site PI between August 2015 and August 2016. Additional physical and chemical characteristics were also measured in surface sediment. Sediment profile images and characteristics of benthic macrofauna communities enabled to estimate the quality of the benthic ecosystem. The highest benthic O2 demand was observed after seasonal anoxia in relation to the important accumulation of reduced chemical species in surface sediment. Interestingly, both pluri-decadal hypoxia and normoxia produced relatively high benthic O2 demand related to a higher accumulation of organic matter and to the presence of reduced chemical species at site dominated by hypoxia, and to the presence of fresher organic matter and active bioturbating macrofaunal communities in normoxic site. The low benthic O2 demand at site seasonally impacted by hypoxia likely resulted from the degraded state of the macrofaunal community and from the lower accumulation of reduced chemical species. The occurrence of hypoxia and anoxia situations in the Berre lagoon was predicted from the competition between kinetics of benthic O2 demand and water column reoxygenation events induced by strong wind. The good agreement between the measured and predicted hypoxia/anoxia occurrence clearly indicates that the chronic deoxygenation events in the Berre lagoon, and the resulting degraded ecological state of the benthic ecosystem are driven both by the benthic O2 demand and by the intensity and duration of the water column stratification.

Geochemical Characteristics of Sediment in Tropical Lake Sentani, Indonesia, Are Influenced by Spatial Differences in Catchment Geology and Water Column Stratification

Physical and (bio)chemical processes in the catchment as well as internal lake processes influence the composition of lacustrine sediments. Lake internal processes are a consequence of reactions and fluxes between sediment, porewater and the water column. Due to its separation into four interconnected sub-basins, Lake Sentani, Papua Province, Indonesia, is a unique tropical lake that reveals a wide range of geochemical conditions. The highly diverse geological catchment causes mineralogical and chemical differentiation of the sediment input into each sub-basin. Also, strong morphological differences between the sub-basins result in a unique water column structure for each sub-basin, ranging from fully mixed to meromictic. Given the strong differences in sediment composition and bottom water chemistry among the four sub-basins, Lake Sentani offers a unique chance to study multiple lacustrine systems under identical climate conditions and with a common surface water chemistry. We used sediment cores and water samples and measured physicochemical water column profiles to reveal the geochemical characteristics of the water column, the sediment and pore water for all four sub-basins of Lake Sentani. The chemical composition of the sediment reveals differentiation among the sub-basins according to their sediment input and water column structure. Catchment lithology mainly affects overall sediment composition, whereas pore water chemistry is also affected by water column structure, which is related to basin morphology and water depth. In the meromictic sub-basins the bottom water and sediment pore water appear to form a single continuous system, whereas in those sub-basins with oxygenated bottom water the sediment-water interface forms a pronounced chemical barrier.

Monsoon and Tropical Climate Forcing on the Physicochemical and Thermocline Characteristics of the Maldives Inner Sea: Insights From Marine Isotope Stages 1–2 and 10–13

AbstractThe Maldivian archipelago, in the equatorial Indian Ocean, provides a unique location to assess long‐term basin‐wide South Asian Monsoon (SAM) processes as well as its response during climatic extremes. This insight is beneficial to better understand future SAM influences on the Maldives Inner Sea physicochemical characteristics and its diverse tropical ecosystems in a warming world. This study uses samples from International Ocean Discovery Program Expedition 359, drilled within the Inner Sea drift deposits. Multiple foraminiferal species (n = 15) and proxies (δ18O, δ13C, and Mg/Ca) are used to assess glacial‐interglacial SAM dynamics influencing the Inner Sea conditions across Marine Isotope Stages (MIS) 1–2 and 10–13. The summer SAM is modulated by insolation and atmospheric CO2 and has a predominant impact on the northern Indian Ocean surface salinity. As with present‐day observations, a strong summer monsoon resulted in large basin‐wide δ18Osw (salinity) gradients during the interglacials. Moreover, at the MIS11 minimum (MIS11c), a recognized analog for the present‐day, sea surface temperatures (SSTs) were warmer than the present with a stronger summer SAM. This led to an expanded surface mixed layer and strong thermocline, resulting in a highly stratified water column and prominent oxygen minimum zone in the Inner Sea during MIS11c. SSTs in the Maldives are projected to increase at the end of this century and based on the reality that current warming (anthropogenically driven) is much faster than seen during MIS11, the Maldivian tropical coral reef and benthic shoal ecosystems will be subject to increasing stress.

Impact of Thermohaline Conditions on Vertical Variability of Optical Properties in the Gulf of Finland (Baltic Sea): Implications for Water Quality Remote Sensing

Vertical variability of inherent optical properties (IOPs) affect the water quality retrievals from remote sensing data. Here, we studied the vertical variability of IOPs and simulated apparent optical properties (AOPs) in the Gulf of Finland (Baltic Sea) under three characteristic (non)stratification conditions. In the case of mixed water column, the vertical variability of optically significant constituents (OSC) and IOPs was relatively small. While in case of stratified water column the IOPs of surface layer were three times higher compared to the IOPs below the thermocline and the IOPs were strongly correlated with the physical parameters (temperature, salinity). Measurements of IOPs in stratified water column showed that the ratio of scattering (b(440)) to absorption (a(440)) changed under the thermocline (b(440)/a(440) < 1) i.e., absorption became the dominant component of attenuation under thermocline while the opposite is true for the upper layer. Simulated (from IOPs) spectral irradiance reflectance (R(λ)) and spectral diffuse attenuation coefficient (Kd(λ)) from deeper layers (below thermocline) have significantly smaller magnitude and smoother shape. This becomes relevant during upwelling events—a common process in the coastal Baltic Sea. We quantified the effect of upwelling on surface water properties using simulated AOPs. The simulated AOPs (from IOPs measurements) showed a decrease of the signal up to 68.8% and an increase of optical depth (z90(λ)) from 2.3 to 4.3 m in the green part of the spectrum in case upwelled water mass reaches the surface. In the coastal waters a vertical decrease of Kd(λ) in the PAR region (400–700 nm) by 6.8% (surface to 20 m depth) was observed, while vertical decrease of chlorophyll-a (Chl-a) and total suspended matter (TSM) was 31.7 and 42.1%, respectively. The ratio R(490)/R(560)≥0.77 indicates also the upwelled water mass. The study showed that upwelling is a process that, in addition to biological activity, horizontal transport of OSC, and temperature changes, alters the optical signal of surface water measured by a remote sensor. Knowledge about the vertical variability of IOPs and AOPs relation to upwelling can help the parametrisation of remote sensing algorithms for retrieving water quality estimates in the coastal regions.

Anoxic in situ production of bacterial GMGTs in the water column and surficial bottom sediments of a meromictic tropical crater lake: Implications for lake paleothermometry

Branched glycerol monoalkyl glycerol tetraethers (brGMGTs) are membrane-spanning lipids which were initially identified in marine settings and have more recently been found in peats and lake sediments. In the latter settings, their abundance relative to that of the branched glycerol dialkyl glycerol tetraethers (brGDGTs) appears to increase under warmer climate conditions. Furthermore, their relative distribution in bottom sediments from an altitudinal transect of East African lakes has been linked to local mean annual air temperature (MAAT). To shed light on their sources and identify potential environmental factors driving their production in lakes, we investigated their occurrence and distribution in the water column, surficial lake-bottom sediments, and catchment soils of Lake Chala, a permanently stratified (meromictic) tropical crater lake in East Africa. Data from the water column comprise both suspended particulate matter (SPM) collected monthly at eight depth intervals between 0 and 80 m for 17 consecutive months, and settling particles collected monthly at a depth of 35 m over a period of 53 months. In catchment soils, brGMGTs are either relatively scarce or absent. In the water column, their occurrence is dominated by four of the seven known brGMGTs and mostly limited to the anoxic zone. Whereas the spatial distribution and concentration of brGMGTs in SPM were strongly influenced by the seasonal cycle of strong water-column stratification alternating with deep mixing, this trend is not consistent on the multi-year timescale of the settling-particles record. Moreover, their relative distribution in both SPM and settling particles did not respond to temperature variation during the studied interval. In contrast to the water column, surficial lake-bottom sediments contain the full suite of known brGMGTs in a distinct and spatially uniform fractional abundance, strongly suggesting that the majority of brGMGT production takes place there. BrGMGT-inferred MAAT for the Chala area reasonably estimates measured MAAT and conforms to the East African lake brGMGT calibration. Our results have important implications for the use of brGMGTs as proxy for MAAT, in that outside of the equatorial zone the presumably greater offset between bottom-water temperature and MAAT may create a bias towards cold-season air temperature. Regardless, the more singular source of brGMGTs in lake sediments is an advantage for paleotemperature reconstruction compared to brGDGTs, which several studies now indicate to have a mixed aquatic and terrestrial source of which the relative contribution cannot easily be disentangled or assumed to have been constant through time.

Tree line advance reduces mixing and oxygen concentrations in arctic-alpine lakes through wind sheltering and organic carbon supply.

Oxygen depletion in lake bottom waters has adverse impacts on ecosystem health including decreased water quality from release of nutrients and reduced substances from sediments, and the reduction of fish growth and reproduction. Depletion occurs when oxygen is consumed during decomposition of organic matter, and oxygen replenishment is limited by water column stratification. Arctic-alpine lakes are often well mixed and oxygenated, but rapid climate change in these regions is an important driver of shifts in catchment vegetation that could affect the mixing and oxygen dynamics of lakes. Here, we analyze high-resolution time series of dissolved oxygen concentration and temperature profiles in 40 Swedish arctic-alpine lakes across the tree line ecotone. The lakes stratified for 1-125days, and during stratification, near-bottom dissolved oxygen concentrations changed by -0.20 to +0.15mgL-1 day-1 , resulting in final concentrations of 1.1-15.5mgL-1 at the end of the longest stratification period. Structural equation modeling revealed that lakes with taller shoreline vegetation relative to lake area had higher dissolved organic carbon concentrations and oxygen consumption rates, but also lower wind speeds and longer stratification periods, and ultimately, lower near-bottom dissolved oxygen concentrations. We use an index of shoreline canopy height and lake area to predict variations among our study lakes in near-bottom dissolved oxygen concentrations at the end of the longest stratification period (R2 =0.41). Upscaling this relationship to 8392 Swedish arctic-alpine lakes revealed that near-bottom dissolved oxygen concentrations drop below 3, 5, and 7mgL-1 in 15%, 32%, and 53% of the lakes and that this proportion is sensitive (5%-22%, 13%-45%, and 29%-69%) to hypothetical tree line shifts observed in the past century or reconstructed for the Holocene (±200m elevation; ±0.5° latitude). Assuming space-for-time substitution, we predict that tree line advance will decrease near-bottom dissolved oxygen concentrations in many arctic-alpine lakes.