Changes In Ocean Environment Research Articles

A review of post-whaling abundance, trends, changes in distribution and migration patterns, and supplementary feeding of Southern Hemisphere humpback whales

Southern Hemisphere humpback whales (Megaptera novaeangliae) were heavily targeted during modern commercial whaling operations, with some 216,000 individuals killed between 1903 and 1973. That impacted the abundance of all the seven breeding stocks of the species. Most of these stocks have been recovering from whaling pressure although the understanding of the current growth rates of some stocks, and how the rates compare across stocks are lacking. Updated information is fundamental for understanding the species’ current status, and to support the review of management plans promoting its protection and recovery, especially considering current changes in ocean environments due to climate change. This work offers a comprehensive overview of the current knowledge on Southern Hemisphere humpback whales breeding stocks’ status. The aim is to provide information on their post-whaling growth trends and changes in distribution and migration patterns. Within that, records of supplementary feeding records (i.e. feeding beyond their formally described feeding grounds) are described. We have also identified knowledge gaps and note that the establishment of research collaborations, as well as standard methodologies for data collection can be important steps for the acquisition of better comparable data sets for the analysis of the current status of humpback whales and to fill such gaps. The compiled information provided can be used as part of an In-Depth Assessment of the species by the International Whaling Commission.

Age offsets between radiocarbon samples and changes in Holocene depositional ocean environments captured in sediment cores near Ube City, Yamaguchi Prefecture, Japan

Radiocarbon ages from contemporaneous samples of varying types can show large age differences (Nakamura et al., 2016; Raymond and Bauer, 2001), and changes in offsets between these ages (δage) within a core have been scarcely examined. Changes in δage have previously suggested disturbances or shifts to the depositional setting (Nakamura et al., 2016; Ishiwa et al., 2021; Yokoyama et al., 2019). Radiocarbon dating of ocean sediment cores from near Ube City, Yamaguchi prefecture, Japan revealed periods of slower and rapid sedimentation which aligned with, respectively, post-Holocene High Stand (HHS, ∼7–4 cal ka BP (Yokoyama, 2021)) conditions and sea level fall, and pre-HHS conditions with rising sea levels.δage patterns implied that the reliability of sample types as depositional age indicators may depend on the depositional environment. Radiocarbon ages and relative sea level (RSL) curves suggested that local inundation may have occurred up to ∼ 200 years earlier than previously reported. In addition to constraining the relative timing of sea transgression and regression during the HHS, this study highlights the need to examine the depositional setting to accurately represent depositional ages.

A 209,000-year-old history of methane seepage activity controlled by multiple factors in the South China Sea

Methane seepage activity caused by gas hydrate dissociation plays an essential role in the global carbon cycle and climate change in Earth's history. However, the triggering mechanisms of gas hydrate dissociation remain elusive because of the lack of a relatively complete history of methane seeps on long-term timescales. Here, we present major and trace elements, stable C–O isotopes, and U–Th dating analyses for cold seep carbonates from drill cores in the South China Sea. Our results reveal a 209,00-year-old history of methane seepage activity, which involves multi-stage changes in seepage intensity and methane source (microbial or thermogenic). Chronological data suggest that hydrostatic pressure reduction and dynamic depositional environment mainly controlled the gas hydrate dynamics in the northwestern South China Sea, which is different from the gas hydrate dissociation in the northeastern South China Sea controlled by bottom water warming. This discrepancy may be caused by the differences in physicochemical conditions (e.g., water depth) in different sea areas. Therefore, the findings of this study help to further clarify the relationship between oceanic hydrate reservoirs and environmental change in other world regions.

Southern Elephant Seals (Mirounga leonina) in the Galapagos Islands and the Eastern Tropical Pacific Amid Ocean Environmental Changes: Towards a Habitat Suitability Index

Southern Elephant Seals (Mirounga leonina) in the Galapagos Islands and the Eastern Tropical Pacific Amid Ocean Environmental Changes: Towards a Habitat Suitability Index

Shallow Water Records of the PETM: Novel Insights From NE India (Eastern Tethys)

AbstractThe Paleocene‐Eocene Thermal Maximum (PETM) is associated with major extinctions in the deep ocean, and significant paleogeographic and ecological changes in surface ocean and terrestrial environments. However, the impact of the associated environmental change on shelf biota is less well understood. Here, we present a new PETM record of a low paleolatitude shallow‐marine carbonate platform from Meghalaya, NE India (eastern Tethys). The biotic assemblage was distinctly different to other Tethyan PETM records dominated by larger benthic foraminifera and calcareous algae both in the Paleocene and Eocene. A change in taxa and forms indicating deeper waters with a concurrent decrease in abundance of shallow water algae suggests a sea‐level rise during the onset of the PETM. The record is lacking the ecological change from corals to larger foraminiferal assemblages and the Lockhartia dominance, characteristic of several other sections in the Tethys. Comparison with a global circulation model (GCM) indicates high regional temperatures in the Thanetian which may have excluded corals from the region. Furthermore, the regional circulation pattern is isolating the site from the wider Paratethys. Our study highlights the need for a diverse global perspective on shallow‐marine response to the PETM and the strength of coupling data to global climate models for interpretation.

Palaeo-environmental evolution and organic matter enrichment of Eopalaeozoic shales, northwestern Tarim Basin, China: Integrated organic and inorganic geochemistry approach

The Eopalaeozoic was a pivotal transitional period following the Neoproterozoic glaciation that witnessed dramatic changes in global ocean environments. The black rock series were extensively sedimented, preserving the original seawater signature. However, due to the scarcity of geological records, the palaeo-environment and organic matter (OM) enrichment in the Tarim Basin, China, are poorly constrained. Herein, Eopalaeozoic shales from typical outcrops in the Kalpin block, northwestern Tarim Basin, have been subjected to integrated organic and inorganic geochemistry research. In the study area, Eopalaeozoic black rock series can be found in the Yuertusi Formation (YUF), Saergan Formation (SAF) and Yingan Formation (YIF) from bottom to top. Although the maturity is moderate (~1.2%), the OM abundance varies greatly vertically. Microscopic observations and biomarkers revealed that the OM source was dominated by diverse planktonic algae. The chemical alteration index indicates a transition from cold and dry climates with low chemical weathering to warm and humid climates with moderate chemical weathering, and subsequently back to cold and dry climates. The stratified water column and euxinic–suboxic conditions are revealed by redox proxies (Mo, enrichment factors, Th/U, MoU covariation pattern, pristane/phytane and gammacerane index). The YUF was deposited in a euxinic environment. Productivity proxies (biogenic Ba, Cu/Al, Ni/Al and carbon isotopes) show that primary productivity was superelevated in the lower YUF and moderate in the other formations. Primary productivity and preservation conditions were heavily influenced by complementary hydrothermal activities and upwelling currents. The hydrothermal activities and upwelling currents provide abundant nutrients and highly reductive environments, creating enormous possibilities for the formation of excellent source rocks with considerable hydrocarbon geological significance.

The mineralization and early diagenesis of deep-sea coral Madrepora oculata

The skeletons of aragonitic corals carry essential information about oceanic environmental changes in the past. However, coral-based geochemical proxies are sensitive to vital effect and post-depositional diagenetic processes, which have not been well understood for deep-sea corals. In this study, we have investigated the mineralization and early diagenetic process of the dendroid stem of a dead deep-sea scleractinian coral sample (Madrepora oculata) collected by Remote Operated Vehicle (ROV) from a seamount in the Caroline Ridge, western Pacific. Despite being only ~270 years old, as dated by the U-series disequilibrium method, this sample already has extensive diagenesis including a visible chalky layer. The outermost chalky layer and the adjacent pristine skeleton of the sample were characterized by multiple mineralogical (SEM, XRD, EBSD, Raman Spectroscopy) and geochemical (ICP-OES, LA-ICP-MS, IR-MS, MC-ICP-MS) techniques. Skeletal stable C and O isotopes display strong linearity, with a trend pointing toward the theoretical isotopic composition of inorganically-precipitated aragonite from local seawater. This observation suggests that the epithecal mineralization of M. oculata is far from equilibrium with seawater and can be quantitatively explained by bio-mineralization models developed for solitary scleractinian corals. We further show that chalking of the outer layer, a common feature in sub-fossilized deep-sea corals, is caused by microbial-driven boring and μm-scale dissolution voids at an average rate of 3.5 μmol/cm2/y (scaled to the skeleton surface) without notable secondary infillings or mineralogical alternations. The higher average δ13C and δ18O of the chalky part than the pristine part might be due to the preferential dissolution of carbonate relatively depleted in 13C and 18O during alteration, and/or reduced isotope fractionation from seawater during coral mineralization of the outer layer. The chalky layer shows significantly higher Mn/Ca and Th/Ca ratios than the pristine part, characterized by Th/Mn ratios similar to that of the dissolved component of local seawater. Our study suggests that the incorporation of external Th into the coral skeleton is most likely associated with organic binding rather than detrital contamination, secondary carbonate mineral formation, adsorption onto surfaces of chalky aragonite or ferromanganese oxides, with important implications for coral U-series geochronology.

Nitrogen isotope evidence for oxygenated upper ocean during the Cryogenian interglacial period

The Cryogenian interglacial period have witnessed dramatic changes in climate, oceanic environment and biological evolution. The nitrogen isotopic composition, as an important biogeochemical proxy, has the potential to track both the nutrient cycling and redox conditions in the past. However, nitrogen isotopic data during this interglacial time is rather limited. Here, we present integrated data for nitrogen isotopes (δ15N), as well as organic carbon isotopes (δ13Corg), iron (Fe) speciation, pyrite morphology and trace elements from the Cryogenian interglacial Datangpo Formation derived from a drill core from South China to figure out the nitrogen cycling and coeval redox states. The results reveal a gradual oxygenation bottom waters along with deepening O2-H2S chemocline from euxinic black shale in the lower unit (LU), to oxic siltstone in the upper unit (UU), which is consistent with the redox variation inferred in previous studies. Sedimentary δ15N values constantly remain positive (mean value> + 4‰) throughout the Datangpo Formation and display an increase from the LU to the UU. We interpreted the positive δ15N values in the LU as a result of denitrification related to the development of euxinic conditions. The further increase towards the UU associated with limited nutrient supply may indicate the existence of a stable nitrate reservoir in the upper water column, which was consistent with an increased oxygenation during the interglacial time. The elevated O2 levels and bioavailable nitrogen in seawater may have further contributed to the appearance of the earliest metazoan. In addition, our data lay good foundation for further global δ15N correlations among other interglacial successions.

Long-term ocean and resource dynamics in a hotspot of climate change

The abundance, distribution, and size of marine species are linked to temperature and nutrient regimes and are profoundly affected by humans through exploitation and climate change. Yet little is known about long-term historical links between ocean environmental changes and resource abundance to provide context for current and potential future trends and inform conservation and management. We synthesize >4000 years of climate and marine ecosystem dynamics in a Northwest Atlantic region currently undergoing rapid changes, the Gulf of Maine and Scotian Shelf. This period spans the late Holocene cooling and recent warming and includes both Indigenous and European influence. We compare environmental records from instrumental, sedimentary, coral, and mollusk archives with ecological records from fossils, archaeological, historical, and modern data, and integrate future model projections of environmental and ecosystem changes. This multidisciplinary synthesis provides insight into multiple reference points and shifting baselines of environmental and ecosystem conditions, and projects a near-future departure from natural climate variability in 2028 for the Scotian Shelf and 2034 for the Gulf of Maine. Our work helps advancing integrative end-to-end modeling to improve the predictive capacity of ecosystem forecasts with climate change. Our results can be used to adjust marine conservation strategies and network planning and adapt ecosystem-based management with climate change.

High-resolution volcanism-induced oceanic environmental change and its impact on organic matter accumulation in the Late Ordovician Upper Yangtze Sea

Volcanism is one of the most important geological processes that impact Earth's surface system. However, little is known about how a volcanic eruption in the past could cause environmental changes in the sea and its impact on organic matter (OM) accumulation due to the difficulties in gaining high-resolution sedimentary records. Here we conducted a high-resolution petrographic study and geochemical analyses including proxies reflecting the influx of volcanic and terrestrial material (Hg, Al, Fe, CIA), redox conditions (Mo, FeHR/FeT, FePy/FeHR), paleoproductivity (Baxs, Cuxs, Sixs), and biological variations (δ13Corg, δ15Norg) on a 25-cm long drilling core bracketing three cycles of volcanic activities from the Upper Ordovician Wufeng Formation in South China. This enables us to find, for the first time, the detailed process of environmental changes and different stages of OM accumulation induced by volcanism. Three nearly identical cycles have been identified, and each of them is comprised of a bentonite bed, a siliceous clay-rich mudstone interval, a radiolarian bed, and a siliceous mudstone interval with intercalations of thin pyrite. The marine redox condition shows the strongest anoxia and even euxinia on the eruption stage and began to decrease with the waning of the volcanic activities shown by Hg/TOC. The volcanic eruption will cause the microbial change that is likely reflecting the shift from nitrogen-fixation bacteria bloom to radiolarian bloom through the variation of bioavailable nitrate and some trace metals such as Cu. Of significance is that the volcanism-induced shift in microbial communities from prokaryotes to eukaryotes bloom is observed to impact the abundance of the sedimentary OM, a critical process leading to the wide occurrence of abundant oil and gas reservoirs in south China.

Status and trends of Arctic Ocean environmental change and its impacts on marine biogeochemistry: Findings from the ArCS project

Ocean observation research theme under ArCS project, “Theme 4: Observational research on Arctic Ocean environmental changes”, aimed to elucidate the status and trends of ongoing Arctic Ocean environmental changes and to evaluate their impacts on Arctic marine ecosystem and the global climate system. For these purposes, we conducted field observations, mooring observations, laboratory experiments, numerical modeling, and international collaborative research focusing on the Pacific Arctic Region (PAR) and from Pan-Arctic point of views. As a result, we have published several scientific studies on environmental changes and their impact on the climate and ecosystem. In this manuscript, we compiled these results with some concluding remarks. We found physical environmental changes of water cycle, sea-ice and ocean conditions, heat transport, and ocean mixing in the Arctic Ocean and surrounding areas. We also examined chemical properties, carbon, cycle, and ocean acidification in the Arctic Ocean. In addition, new findings regarding impacts of sea-ice reduction to primary productivities were published. For public outreach of Arctic research, we were able to develop an educational tool (a board game named “The Arctic”) in collaboration with Themes 6 and 7.

Paleoenvironmental implications of Sr and Nd isotopes variability over the past 48 ka from the southern Sea of Japan

Millennial scale variations of terrigenous provenance in marine realm are closely related to regional environment and climate changes. Therefore, a wealth of information of past environment and climate can be constrained via fingerprinting sediment provenance. The Sea of Japan is a unique marginal sea in the North Pacific due to its high sill and distinct thermohaline circulation. The modern hydrography in the Sea of Japan is mainly affected by the East Asian Monsoon and Tsushima Warm Current, one branch of the Kuroshio Current. The Sea of Japan communicates with neighboring seas through four shallow and narrow straits, indicating great effects of global eustatic sea level change on its environment over glacial-interglacial cycles. Here we examine the terrigenous provenance in fine-grained fraction (<63 μm) of core KCES1, located near one end of the Tsushima Strait of the Sea of Japan over the last 48 ka, using radiogenic isotopes of strontium (Sr) and neodymium (Nd). Our data suggest that the terrigenous provenance in core KCES1 was mainly derived from the Yangtze River after 7 ka and a mixture of Yangtze and Yellow Rivers during the last glacial and deglacial periods. Notably, pronounced negative excursions of εNd values at HS1 were attributed to minor additions of unradiogenic Nd contribution from China-Korea cratonic hinterland. A binary mixing model further reveals that >85% terrigenous material is derived from the Yangtze and Yellow Rivers over the last 48 ka. Moreover, abrupt variations in sediment provenance occurred at ~18 ka and ~ 7 ka, which coincide with variations in oceanic surface circulation and deep ventilation recorded in the Sea of Japan. We suggest that paleo-Tsushima Warm Current invaded into the Sea of Japan with reopening of the Tsushima Strait at HS1 and the Tsushima Warm Current substantially entered the Sea of Japan after 7 ka due to intensified Kuroshio Current and rising eustatic sea level. The inflow of Tsushima Warm Current gives rise to a range of changes in surface hydrography, deep ventilation, ecological communities and productivity and sediment texture. The combination of fluxes of paleo-rivers and the intensity of Kuroshio Current, which are closely tied to the eustatic sea level and the East Asian Monsoon, plays a key role in controlling the variations in sediment provenance in the Ulleung Basin. Our study provides unique insight into the tight coupling between changes in sediment provenance and oceanic environment over the last 48 ka in the Sea of Japan.

Extracellular Vesicles and Post-Translational Protein Deimination Signatures in Mollusca-The Blue Mussel (Mytilus edulis), Soft Shell Clam (Mya arenaria), Eastern Oyster (Crassostrea virginica) and Atlantic Jacknife Clam (Ensis leei).

Simple SummaryOysters and clams form an important component of the food chain and food security and are of considerable commercial value worldwide. They are affected by pollution and climate change, as well as a range of infections, some of which are opportunistic. For aquaculture purposes they are furthermore of great commercial value and changes in their immune responses can also serve as indicators of changes in ocean environments. Therefore, studies into understanding new factors in their immune systems may aid new biomarker discovery and are of considerable value. This study assessed new biomarkers relating to changes in protein function in four economically important marine molluscs, the blue mussel, soft shell clam, Eastern oyster, and Atlantic jacknife clam. These findings indicate novel regulatory mechanisms of important metabolic and immunology related pathways in these mollusks. The findings provide new understanding to how these pathways function in diverse ways in different animal species as well as aiding new biomarker discovery for Mollusca aquaculture.Oysters and clams are important for food security and of commercial value worldwide. They are affected by anthropogenic changes and opportunistic pathogens and can be indicators of changes in ocean environments. Therefore, studies into biomarker discovery are of considerable value. This study aimed at assessing extracellular vesicle (EV) signatures and post-translational protein deimination profiles of hemolymph from four commercially valuable Mollusca species, the blue mussel (Mytilus edulis), soft shell clam (Mya arenaria), Eastern oyster (Crassostrea virginica), and Atlantic jacknife clam (Ensis leei). EVs form part of cellular communication by transporting protein and genetic cargo and play roles in immunity and host–pathogen interactions. Protein deimination is a post-translational modification caused by peptidylarginine deiminases (PADs), and can facilitate protein moonlighting in health and disease. The current study identified hemolymph-EV profiles in the four Mollusca species, revealing some species differences. Deiminated protein candidates differed in hemolymph between the species, with some common targets between all four species (e.g., histone H3 and H4, actin, and GAPDH), while other hits were species-specific; in blue mussel these included heavy metal binding protein, heat shock proteins 60 and 90, 2-phospho-D-glycerate hydrolyase, GTP cyclohydrolase feedback regulatory protein, sodium/potassium-transporting ATPase, and fibrinogen domain containing protein. In soft shell clam specific deimination hits included dynein, MCM3-associated protein, and SCRN. In Eastern oyster specific deimination hits included muscle LIM protein, beta-1,3-glucan-binding protein, myosin heavy chain, thaumatin-like protein, vWFA domain-containing protein, BTB domain-containing protein, amylase, and beta-catenin. Deiminated proteins specific to Atlantic jackknife clam included nacre c1q domain-containing protein and PDZ domain-containing protein In addition, some proteins were common as deiminated targets between two or three of the Bivalvia species under study (e.g., EP protein, C1q domain containing protein, histone H2B, tubulin, elongation factor 1-alpha, dominin, extracellular superoxide dismutase). Protein interaction network analysis for the deiminated protein hits revealed major pathways relevant for immunity and metabolism, providing novel insights into post-translational regulation via deimination. The study contributes to EV characterization in diverse taxa and understanding of roles for PAD-mediated regulation of immune and metabolic pathways throughout phylogeny.

Revisiting the phylogeny of the genus Lolliguncula Steenstrup 1881 improves understanding of their biogeography and proves the validity of Lolliguncula argus Brakoniecki & Roper, 1985

The biogeography of American loliginid squids has been improved in recent years, but certain key taxa have been missing. Given that the most accurate phylogenies and estimates of divergence times of common ancestors depend heavily on good taxonomic coverage we have reanalyzed the genus Lolliguncula in light of new samples that increase the geographic and taxonomic coverage. New sequences were produced using standard methods to update an existing dataset for COI, 16S and Rhodopsin markers. Data was analyzed using various species delimitation methods, rigorous phylogenetic analyses and estimates of divergence times between clades. Within Lolliguncula we recover five monophyletic lineages that relate to the known species L. argus, L. diomedeae, L. panamensis, L. brevis North Atlantic and L. brevis South Atlantic. Except when using low divergence thresholds in ABGD, species delimitation methods only identify four of these lineages as distinct species, grouping L. argus and L. diomedeae as a single species. However, considering the reciprocal monophyly, recent divergence time estimate and morphological diagnoses we refrain from synonymizing L. argus within L. diomedeae, considering them very recently diverged species. The biogeography of the American loliginids is discussed, wherein basal cladogenesis in both Lolliguncula and Doryteuthis occur between the Atlantic and Pacific about 45 mya, with subsequent speciation around 20 mya associated with seafloor changes during the formation of the Caribbean. The recent speciation between L. argus and L. diomedeae is associated to oceanic environmental changes associated with glaciation, deep sea cooling and tropical upwelling.

Lower Salinity Leads to Improved Physiological Performance in the Coccolithophorid Emiliania huxleyi, Which Partly Ameliorates the Effects of Ocean Acidification

While seawater acidification induced by elevated CO2 is known to impact coccolithophores, the effects in combination with decreased salinity caused by sea ice melting and/or hydrological events have not been documented. Here we show the combined effects of seawater acidification and reduced salinity on growth, photosynthesis and calcification of Emiliania huxleyi grown at 2 CO2 concentrations (low CO2 LC：400 μatm; high CO2 HC：1000 μatm) and 3 levels of salinity (25, 30 and 35‰). A decrease of salinity from 35 to 25‰ increased growth rate, cell size and photosynthetic performance under both LC and HC. Calcification rates were higher in the LC compared to the HC-grown cells only at 25‰ salinity, with insignificant changes under 30 and 35‰ levels. Since salinity and OA treatment did not bring about interactive effects on calcification, changes in calcification are attributed to the elevated dissolved inorganic carbon concentration, with higher ratios of calcification to photosynthesis in the cells grown under 35‰ compared with those grown at 25‰. In contrast, photosynthetic carbon fixation increased almost linearly with decreasing salinity, regardless of the pCO2 treatments. When subjected to short-term exposure to high light, the low-salinity-grown cells showed the highest photochemical effective quantum yield with the highest repair rate, though the HC treatment enhanced the PSII damage rate. Our results suggest that Emiliania huxleyi can tolerate low salinity plus acidification conditions by up-regulating its photosynthetic performance, but shows a relatively insensitive calcification response, which may help it better adapt to future ocean global environmental changes, especially in the coastal areas of high latitudes.

Editorial: North Pacific Environment and Paleoclimate From the Late Pleistocene to Present

The vast area of the North Pacific, spanning ~55° longitude, represents a challenge for documenting and understanding the geologic history of ocean, atmosphere, and terrestrial environmental change. This special issue highlights site-specific analyses to address various questions and provides clues that arise in response to continued North Pacific warming today in a rapidly changing climate (Figure 1). The emergence of new methods and novel application of existing methods serve to enhance our fundamental understanding of natural modes of climate variability driven by ocean and atmospheric circulation patterns, including the role of external (e.g., insolation) vs. internal forcing [e.g., El Nino Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO)]. Included in this volume are geographically diverse studies that present new and traditional methods for the interpretation of climate records drawn from pollen, macrofossils, tree rings, diatoms, grain size, and glacial studies. Many provide new geographic comparisons, such as Kamchatka and Hawaii, or emerge from novel geologic archives and environments, such as the marine shelf, a thermokarst lake, and a fossil forest.

Oceanic environment changes caused the Late Ordovician extinction: evidence from geochemical and Nd isotopic composition in the Yangtze area, South China

AbstractThe Ordovician–Silurian (O–S) transition was a critical interval in geological history. Multiple geochemical methods are used to explore the changes in oceanic environment. The Nd isotopic compositions in the Yangtze Sea are controlled by two sources: the continental erosion and the Panthalassa Ocean. HighεNd(t) values during the Katian, late Hirnantian and Rhuddanian intervals are associated with the high sea level, which resulted in less terrestrial input based on the low Ti/Al and Zr/Al ratios. In contrast, lowεNd(t) values during the early Hirnantian interval are related to the sea-level fall; in this case, the exposure of submarine highs and the growth of Yangtze Oldlands could lead to more continental materials being transported into the Yangtze Sea based on high Ti/Al and Zr/Al ratios. In addition, the negativeεNd(t) excursion can also be attributed to the weak circulation between the Yangtze Sea and Panthalassa Ocean when sea level was low. Furthermore, the sea-level eustacy plays a significant role in the changes in redox water conditions. The redox indices, mainly UEF, Ce/Ce* and Corg/PT, across the O–S transition show a predominance of anoxic ocean over the Yangtze Sea during the Katian, late Hirnantian and Rhuddanian intervals, and an oxygenated episode was briefly introduced during the early Hirnantian period because of the fall in sea level. The Late Ordovician biotic crisis was marked by two-phase extinction events, and the change in sea level and redox chemistry may be the important kill mechanisms.

Disentangling Environmental Drivers of Phytoplankton Biomass off Western Iberia

Phytoplankton are the main primary producers in marine ecosystems, supporting important food webs. They are recognized as important indicators of environmental changes in oceans and coastal waters. Ocean colour remote sensing has been extensively used to study phytoplankton throughout the world, yet there is still much to understand in terms of what influences phytoplankton variability at regional scales. The main aim of this study was to investigate the drivers of phytoplankton variability in the Western Iberian Coast (WIC), for the period 1998-2016. Satellite chlorophyll a (CHL) data was acquired from the Copernicus Marine Environment Monitoring Service (CMEMS). A positive annual trend of CHL was observed at the northern coastal WIC, near Galicia, and a negative trend was observed in southern areas, near the Gulf of Cadiz. An empirical Orthogonal Function (EOF) analysis was implemented to identify regions with similar patterns of CHL variability. Six regions were obtained. A set of climate indices, satellite and model variables were then used as environmental predictors in Generalized Additive Models (GAMs) that explained between 18.0 to 49.3% of the total variance of CHL anomalies calculated from a detrended and deseasoned dataset. In the northern oceanic region positive anomalies were linked to high North Atlantic Oscillation (NAO) values and negative anomalies to high mixed layer depths. In the southern oceanic region, positive CHL anomalies were found to be associated with high concentrations of nitrogen, that may indicate nitrogen limitation. Phytoplankton in coastal areas were found to respond to basin-wide (e.g. Atlantic Multidecadal Oscillation-AMO) and coastal processes (e.g. upwelling and continental runoff), yielding positive anomalies with low salinity values. In very small coastal areas near major rivers, such as Douro and Guadalquivir, the positive response of phytoplankton to increased nitrogen concentrations and decreased salinity was evident. Considering the changes in climate expected for this region, related to the decrease in precipitation and increase in summer temperatures, as well as some apparent weakening of upwelling, possible significant impacts on the phytoplankton community can be anticipated. These results are therefore also relevant for environmental management, especially in the context of the European Marine Strategy Framework Directive (MSFD).

Anthropogenic effects on tropical oceanic climate change and variability: An insight from the South China Sea over the past 2000 years

To better understand the effect of human activities on tropical oceanic climate change, existing coral-based paleoclimate data from the northern South China Sea (SCS) for several representative climate periods over the past 2000 years were synthesized and reanalyzed. The sea surface temperature (SST) during the current warm period (CWP), in which the climate has been influenced by human activities, exhibits larger change amplitude and higher oscillation frequency than during periods in which the climate was controlled by natural forcings. However, the variabilities of the sea water δ18O (δ18Osw) during the four climate periods over the past 2000 years examined here are somewhat complicated to decipher. The variation amplitudes are variable but the interannual oscillations are similar during different periods. Even so, high variability during the CWP may still suggest the anthropogenic effects on tropical oceanic hydroclimate. This may implies that the anthropogenic effects intensify the tropical SST and hydrological variation both in amplitude and frequency. The decoupling between coral δ13C and total solar insolation during the CWP may be a result of the oceanic δ13C Suess effect from the addition of anthropogenic 12CO2 to the surface ocean. This suggests that increased anthropogenic CO2 caused the main carbon source and the utilization process of coral reefs during calcification to change. To accurately assess the influence of human activities on oceanic climate and environmental change, the effects of anthropogenic CO2 should be monitored continuously over a longer time frame.

Biological modulation in the seasonal variation of dissolved oxygen concentration in the upper Japan Sea

Dissolved oxygen (DO) can be influenced by both physical and biogeochemical conditions of the ocean and is hence an important index for predicting changes in ocean environments. However, the relationship between biogeochemical activities and the DO concentration has not been quantified clearly in the Japan Sea (JS). In this study, we developed a coupled physical-biogeochemical model and clarified the contributions of biological processes to the DO content in the upper JS. We conducted two numerical experiments: with and without the biogeochemical compartments. Taking the biogeochemical factors into account gave better estimations for the spring-to-autumn seasonal migration of the subsurface DO maximum. In the euphotic zone, high photosynthetic production increases the DO concentration from spring to autumn. The strongest positive effect of biological processes on DO concentrations appears in summer at a depth of 30 m and is more than 0.5 ml l−1. As summer progresses to autumn, the vertical pattern of DO concentration in the Tsushima Warm Current (TWC) region shows minima at depths between 50 and 100 m, which is caused by the intrusion of low DO water from the East China Sea. The strongest negative effect on DO concentrations mainly due to the decomposition of detritus occurs in the TWC region below 150 m depth because the water is poorly ventilated by the relatively weak winter convection.