Bottom Water Temperature Research Articles

Lunar Periodicity in Reproduction of Senatorial Scallop (Chlamys Senatoria Gmelin, 1791) in Asid Gulf, Masbate, Philippines

Abstract In Masbate, senatorial scallop Chlamys senatoria is one of the five commercially important species of scallop present in Asid Gulf, but its basic biology is poorly understood in the Philippines. To provide biological information for future conservation and management, the reproductive rhythm of senatorial scallop in relation to lunar phase was investigated from October 2016 to March 2017. Scallops were collected daily from the fishers’ catches and were measured, dissected, and processed for histological analysis; whereas, fecundity and size of eggs (diameter) were also determined. Ecological parameter such as bottom water temperature was monitored during the sampling period. Results showed that C. senatoria is a highly fecund species, which ranges from 8.3 X 105 to 2.1 X 106 oocytes per female and its sizes (oocytes) ranges from 53.8 µm to 72.5 µm while Gonado-somatic index (GSI) ranges from 3.2 to 7.1. Fecundity, egg diameter, and the GSI decreased during new moon (NM) and full moon (FM). GSI indicates that the onset of spawning activity commences during NM and FM. Environmental parameters that were monitored showed significant variations in relation to lunar phase. Spawning of scallop C. senatoria follows a lunar reproductive pattern, which was initiated and influenced by the variations of different factors such as temperature, water amplitude, and light as the effect of lunar/tidal cycles. These findings indicate that reproduction and necessarily spawning in the scallop are related to factors associated with lunar phases. Strategies for managing the resource can be formulated using the temporal dimension of lunar phases.

Mg/Ca ratios in ostracode genera Sarsicytheridea and Paracyprideis: A potential paleotemperature proxy for Arctic and subarctic continental shelf and slope waters

We evaluate the potential utility of Mg/Ca in the ostracode genera Sarsicytheridea and Paracyprideis as a paleotemperature proxy for continental shelf and upper slope waters of the Arctic Ocean and adjacent seas. Using sediment core-top and surface sediment samples, shells of three species, S. bradii, S. punctillata, and P. pseudopunctillata, were analyzed from Arctic Ocean sites in water depth from 7 to 200 m and bottom-water temperatures (BWT) of −1.5 °C to 12 °C. Mg/Ca values range from 4 to 9 mmol/mol. These results are in excellent agreement with the range of Mg/Ca obtained in a previous study (Ingram, 1998) of shells of Sarsicytheridea from the sub-polar North Atlantic Ocean region with BWT ranging from 1 to 11.4 °C. Unlike this study of Sarsicytheridea, and work on the Arctic ostracode marine genus Krithe showing strong correlations between Mg/Ca and BWT, analysis of Mg/Ca in this study are not correlated with BWT below 0 °C. We hypothesize that this primarily has to do with uncertainty regarding the actual ambient BWT at the time of shell secretion. As with many continental shelf sites, individual sites used in the study have large interannual and seasonal variations in BWT (≥5–8 °C). Thus, there is difficulty in assigning a shell secretion temperature and assessing the utility of Sarsicytheridea Mg/Ca ratios as a paleotemperature proxy below 0 °C. The calibration of Mg/Ca results from temperatures above 0 °C justifies continued evaluation of this potential paleothermometer for Arctic continental shelf and upper slope environments.

Patterns of species and functional diversity of macrofaunal assemblages and the bioassessment of benthic ecological quality status in the southern Yellow Sea

Macrofauna are useful indicators of benthic ecological quality status, which were studied in summer and autumn of 2020 in the southern Yellow Sea, China. A total of 134 species were identified, and polychaetes represented the most dominant group. The dominant species across both seasons were Ophiura sarsii vadicola, Trigonothracia jinxingae, Heteromastus filiforms, Ninoё palmata, Thyasira tokunagai, Sigambra hanaokai, Ehlersileanira incisa hwanghaiensis, and Portlandia japonica. The average values of macrofaunal abundance and biomass were 386.68 ind./m2 and 28.21 g/m2, respectively. Species diversity during the autumn was slightly higher than that in the summer. Seasonal variations in functional diversity were also observed. BIOENV results indicated that water depth, bottom water temperature, and sediment phaeophorbide content was the optimal combination of parameters to explain macrofaunal assemblage variation. Bioassessment results, based on H’, AMBI, and M-AMBI, showed that most sites in the southern Yellow Sea could be classed as moderate or good status.

Oxygen Isotope Offsets in Deep-Water Benthic Foraminifera

ABSTRACT Despite the extensive use of the benthic foraminiferal oxygen isotope composition (δ18O) as a proxy for paleoclimatic reconstructions, uncertainties remain regarding the consistency of interspecies offsets and the environmental factors controlling 18O fractionation. We investigated δ18O offsets of some frequently used Uvigerina, Bulimina, and Cibicidoides species in core top samples from different hydrographic and sedimentary regimes in the South China Sea, Makassar Strait, and Timor Strait/Eastern Indian Ocean. The δ18O values of the epifaunal taxa Cibicidoides mundulus and Cibicidoides wuellerstorfi showed no significant offset in all investigated regions, whereas shallow infaunal Cibicidoides species exhibited higher variability and were less reliable. We found no offsets between species of Uvigerina and Bulimina and assume that these genera can be measured together and/or substituted. Our results show that epifaunal taxa are close to equilibrium with ambient seawater and thus provide more reliable records of past ice volume and/or bottom water temperature variations than infaunal taxa. Offsets among equilibrium calcite, epifaunal taxa, and infaunal taxa are not constant “vital effects” but are influenced by changing gradients in bottom to pore water pH and carbonate ion concentrations that depend on deep-water ventilation and export flux of particulate carbonate and organic carbon. Offsets between epifaunal and infaunal taxa varied between 0.58 and 0.73‰, depending on regional bottom and pore water conditions. Our findings highlight the importance of regional and temporal variations in organic carbon flux/degradation and dissolution of calcite that may lead to slight under- or overestimates of the amplitude of δ18O fluctuations, especially during times of rapidly changing calcite-saturation of bottom and pore water.

Modelling of the gas hydrate potential in Svalbard’s fjords

Large amounts of methane are trapped as natural gas hydrate (NGH) in the sediments of the Arctic. Unlike NGH provinces offshore west of Svalbard (Vestnesa Ridge), NGH potential in Svalbard’s fjords and near-shore environment is poorly constrained. In this study we modelled the NGH stability zone (GHSZ) to determine the NGH formation potential within the fjords of Svalbard. We applied a nearest neighbour interpolation method to dynamically derive statistical bottom-water temperature (BWT) trends from available CTD data. The BWT trends along with available geothermal gradient data constrained Svalbard-wide sub-bottom thermobaric models suitable for GHSZ modelling in the near subsurface. Analyses of source rock and fluid seepage data in Isfjorden, including 15 newly identified acoustic gas flares, indicate an active petroleum system with fluid migration reaching the seafloor with significant contributions of higher-order hydrocarbons to the gas feed. A GHSZ is predicted for most fjords at mean BWT conditions and 95:5 methane:ethane gas compositions. Suitable conditions for pure methane NGH formation are only met in the deepest parts of Isfjorden, Hinlopenstretet, Kross- and Kongsfjorden, and Rijpfjorden. Temporal constraints implemented along the well-defined Isfjorden transect indicated a notable negative response to water column warming. The predicted GHSZ across Svalbard’s fjords is likely to disappear over the next few decades.

Gradually Cooling of the Yellow Sea Warm Current Driven by Tropical Pacific Subsurface Water Temperature Changes Over the Past 5 kyr

AbstractThe integrated effects of ocean‐atmosphere dynamics on the temperature evolution in the western North Pacific marginal seas have remained elusive. In order to study mechanisms controlling southern Yellow Sea (YS) temperature changes, bottom water temperature (BWT) changes were reconstructed for the last 8.8 kyr by using the TEXL86 index, which archives temperature signal of the winter season Yellow Sea Warm Current. Our results reveal a series of abrupt multi‐centennial to millennial scale BWT changes (∼1°C), superimposed on a gradual long‐term cooling (>3°C) trend starting from ∼5 ka to the present. The YS BWT changes are positively correlated with subsurface water temperature changes in the Western Pacific Warm Pool (WPWP). The Western Pacific Warm Pool subsurface water cooling signal was most likely transmitted by the Kuroshio Current into the southern YS, highlighting the role of WPWP in influencing thermodynamics of the extratropical regions during the Holocene.

Historical and Future Projected Warming of Antarctic Shelf Bottom Water in CMIP6 Models

AbstractUnderstanding warming on the Antarctic shelf is critical for projecting changes in Antarctic ice shelves and ice sheets. Here we assess Antarctic Shelf Bottom Water (ASBW) temperature mean‐state and trends in CMIP6 models. While CMIP6 models do not resolve ice shelves, future shelf water warming will impact ice shelf vulnerability. The CMIP6 multi‐model mean zonal temperature structure and mean‐state ASBW spatial pattern resemble observations, although there is considerable spread across the models and a multi‐model mean warm bias. The multi‐model mean projects an average ASBW warming of 0.36°C (interdecile range 0.07°C–0.60°C) under SSP245 and 0.62°C (interdecile range 0.16°C–0.95°C) under SSP585 by 2100, emphasizing the influence future emissions have on shelf water warming around Antarctica. Changes in the transport of Circumpolar Deep Water onto the shelf associated with changes in the Southern Annular Mode, as well as Circumpolar Deep Water warming, are predicted to conspire to warm ASBW in the future.

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.

Biogeography and ecology of Ostracoda in the U.S. northern Bering, Chukchi, and Beaufort Seas.

Ostracoda (bivalved Crustacea) comprise a significant part of the benthic meiofauna in the Pacific-Arctic region, including more than 50 species, many with identifiable ecological tolerances. These species hold potential as useful indicators of past and future ecosystem changes. In this study, we examined benthic ostracodes from nearly 300 surface sediment samples, >34,000 specimens, from three regions-the northern Bering, Chukchi and Beaufort Seas-to establish species' ecology and distribution. Samples were collected during various sampling programs from 1970 through 2018 on the continental shelves at 20 to ~100m water depth. Ordination analyses using species' relative frequencies identified six species, Normanicythere leioderma, Sarsicytheridea bradii, Paracyprideis pseudopunctillata, Semicytherura complanata, Schizocythere ikeyai, and Munseyella mananensis, as having diagnostic habitat ranges in bottom water temperatures, salinities, sediment substrates and/or food sources. Species relative abundances and distributions can be used to infer past bottom environmental conditions in sediment archives for paleo-reconstructions and to characterize potential changes in Pacific-Arctic ecosystems in future sampling studies. Statistical analyses further showed ostracode assemblages grouped by the summer water masses influencing the area. Offshore-to-nearshore transects of samples across different water masses showed that complex water mass characteristics, such as bottom temperature, productivity, as well as sediment texture, influenced the relative frequencies of ostracode species over small spatial scales. On the larger biogeographic scale, synoptic ordination analyses showed dominant species-N. leioderma (Bering Sea), P. pseudopunctillata (offshore Chukchi and Beaufort Seas), and S. bradii (all regions)-remained fairly constant over recent decades. However, during 2013-2018, northern Pacific species M. mananensis and S. ikeyai increased in abundance by small but significant proportions in the Chukchi Sea region compared to earlier years. It is yet unclear if these assemblage changes signify a meiofaunal response to changing water mass properties and if this trend will continue in the future. Our new ecological data on ostracode species and biogeography suggest these hypotheses can be tested with future benthic monitoring efforts.

Ocean-Bottom Seismology of Glacial Earthquakes: The Concept, Lessons Learned, and Mind the Sediments

Abstract About 70% of Earth’s surface is covered by ocean, for which seismic observations are challenging. Seafloor seismology overcame this fundamental difficulty and radically transformed the earth sciences, as it expanded the coverage of seismic networks and revealed otherwise inaccessible features. At the same time, there has been a recent increase in the number of studies on cryoseismology. These have yielded multiple discoveries but are limited primarily to land and ice-surface receivers. Near ice calving fronts, such surface stations are noisy, primarily due to crevassing and wind, are hazardous to maintain, and can be lost due to iceberg calving. To circumvent these issues, we have applied ocean-bottom seismology to the calving front of a tidewater glacier in northwest Greenland. We present details of this experiment, and describe the technical challenges, noise analysis, and examples of recorded data. This includes tide-modulated seismicity with thousands of icequakes per day and the first near-source (∼200–640 m) underwater record of a major kilometer-scale calving event in Greenland, which generated a glacial earthquake that was detectable ∼420 km away. We also identified a decrease in bottom-water temperature, presumably due to modified water stratification driven by extreme Greenland glacial melting, at the end of July 2019. Importantly, we identify glacial sediments as the key reason for the anomalously long (∼9.7 hr) delay in the sensor release from the fjord seafloor. Our study demonstrates a methodology to undertake innovative, interdisciplinary, near-source studies on glacier basal sliding, calving, and marine-mammal vocalizations.

Environmental conditions, diel period, and fish size influence the horizontal and vertical movements of red snapper

Most demersal fishes are difficult to observe and track due to methodological and analytical constraints. We used an acoustic positioning system to elucidate the horizontal and vertical movements of 44 red snapper (Lutjanus campechanus) off North Carolina, USA, in 2019. Mean movement rate and distance off bottom varied by individual, with larger red snapper generally moving faster and spending more time farther off the bottom than smaller individuals. We used generalized additive mixed models that accounted for temporal autocorrelation in the data to show that mean hourly red snapper movement rate was lower during the day than at night and was negatively related to bottom water temperature. Moreover, red snapper spent more time off the bottom during the day than at night, and vertical movements were mostly related to bottom upwelling events that sporadically occurred in May–July. Our results and previous observations suggest that red snapper feed primarily on benthic organisms at night, and display diel vertical migration (i.e., thermotaxis) up to warmer waters (when present) during the day to aid digestive efficiency. Movement is a central organizing feature in ecology, and the sustainable management of fish will benefit from a better understanding of the timing and causes of fish movement.

Deep-ocean circulation in the North Atlantic during the Plio-Pleistocene intensification of Northern Hemisphere Glaciation (~2.65–2.4 Ma)

Through transporting heat around the world and thereby regulating global climate, ocean circulation is an integral part of Earth's climate system that likely changed substantially on glacial-interglacial timescales in the deep North Atlantic Ocean. However, quantitative records on deep-water dynamics in the North Atlantic older than ~1 Myr are sparse and typically of low temporal resolution. Here we provide a new record on northern- versus southern-sourced waters in the deep North Atlantic at a yet unprecedented temporal resolution. Our record is based on a novel approach (Atlantic-Pacific bottom-water temperature (BWT) difference) for an interval comprising the onset of larger-scale glaciations in the Northern Hemisphere, the Plio-Pleistocene transition (~2.65–2.4 Ma; Marine Isotope Stages G1–95). We thus have generated a new, millennial-scale-resolution BWT record based on benthic foraminiferal Mg/Ca from Ocean Drilling Program Site 849 in the East Pacific backed up with a new neodymium-isotope record on glacial-interglacial-scale resolution. The difference between our new BWT record (Site 849) and previously available BWTs from Integrated Ocean Drilling Program Site U1313 in the North Atlantic was used to decipher changes between southern- and northern-sourced waters in the deep North Atlantic. In accordance with previous studies, we document an increased influence of deep waters from high southern latitudes during glacials at the expense of northern-sourced waters. Perhaps even more importantly, our new record is highly relevant for the determination of geochemically-based sea-level records from the deep North Atlantic, which was yet limited by quantitative estimates of deep-water masses at a sufficient, (sub-)millennial-scale resolution.

Projecting regions of North Atlantic right whale, Eubalaena glacialis, habitat suitability in the Gulf of Maine for the year 2050

North Atlantic right whales (Eubalaena glacialis) are critically endangered, and recent changes in distribution patterns have been a major management challenge. Understanding the role that environmental conditions play in habitat suitability helps to determine the regions in need of monitoring or protection for conservation of the species, particularly as climate change shifts suitable habitat. This study used three species distribution modeling algorithms, together with historical whale abundance data (1993–2009) and environmental covariate data, to build monthly ensemble models of past E. glacialis habitat suitability in the Gulf of Maine. The model was projected onto the year 2050 for a range of climate scenarios. Specifically, the distribution of the species was modeled using generalized additive models, boosted regression trees, and artificial neural networks, with environmental covariates that included sea surface temperature, bottom water temperature, bathymetry, a modeled Calanus finmarchicus habitat index, and chlorophyll. Year-2050 projections used downscaled climate anomaly fields from Representative Concentration Pathway 4.5 and 8.5. The relative contribution of each covariate changed seasonally, with an increase in the importance of bottom temperature and C. finmarchicus in the summer, when model performance was highest. A negative correlation was observed between model performance and sea surface temperature contribution. The 2050 projections indicated decreased habitat suitability across the Gulf of Maine in the period from July through October, with the exception of narrow bands along the Scotian Shelf. The results suggest that regions outside of the current areas of conservation focus may become increasingly important habitats for E. glacialis under future climate scenarios.

Biogenic matter content in marine sediments in the vicinity of the Antarctic Peninsula: Recent sedimentary conditions under a diverse environment of production, transport, selective preservation and accumulation

Burial fluxes of organic carbon and biogenic silica were determined in 17 continental shelf sediment cores collected from the northern Weddell Sea, the Bransfield Strait, and the southern Drake Passage. Coring sites included open-shelf stations as well as slope and glacial trough environments, with water depths varying from 220 to 760 m. Apparent 210Pb accumulation rates from these cores ranged from 0.04 g m−2y−1 to 0.21 g m−2y−1 (1 to 3 mm y−1), with organic carbon burial rates ranging from 3 to 15 g OC m−2y−1 and biogenic silica accumulation rates ranging from 15 to 126 g SiO2 m−2y−1. OC contents below the surface mixed layer ranged from 0.26 to 1.51 wt. % (avg. 0.64 %). Biogenic silica contents at depth ranged from 2.3 to 11.2 wt. % (avg. 7.5%), with an average bSi/OC ratio (wt. %/wt. %) at depth of 12. Annual OC primary production rates and biogenic silica production rates in the euphotic zone were estimated from satellite chlorophyll-a data in the literature and from a seasonal model for biogenic particle export from surface waters. Based on these biogeochemical data, preservation efficiencies (i.e., mass burial rate/water column production rate) were calculated for organic carbon and biogenic silica. These preservation efficiency values ranged from 2 to 18% (avg. 9%) for OC and 8 to 106% (avg. 54%) for bSi. These relatively high preservation efficiencies resulted from extensive lateral sediment focusing (210Pb Psi (Ψ) values [burial flux/water column production rate] ranging from 2 to 33; avg. of 16), cold bottom water temperatures (2 to −2°C), and relatively high biogenic Si and OC production rates in the euphotic zone. The enhanced preservation efficiency for bSi relative to OC (i.e., 54% vs. 9%) in these Antarctic settings is consistent with the change in the phytoplankton bSi/OC (wt. %/wt. %) value of 2 for this area to the burial bSi/OC value of 12. Excess 210Pb activities in surface sediments varied from 4 to 47 dpm g−1. The surface mixed layer in the seabed varied in thickness from 0 to 4 cm. The penetration of excess 210Pb into these Antarctic Peninsula sediments varied from 6 to 28 cm (avg. 18 cm). The inventory of excess 210Pb in the seabed varied from 13 to 230 dpm cm−2 (avg. 110 dpm cm−2). Although 210Pb was the only radionuclide measured in this study, “apparent” 210Pb sediment accumulation rate (SAR) values from these 17 cores (assuming that deep bioturbation is negligible) are believed to be accurate SAR values because of good agreement between 210Pb and 14C chronologies from nearby cores reported in the literature.

Characteristics of macrofaunal assemblages and their relationships with environmental factors in a semi-enclosed bay

Characteristics of macrofaunal assemblages and their relationships with environmental factors in a semi-enclosed bay were studied seasonally in China. A total of 251 species were identified, including polychaetes (104 species), crustaceans (73 species), mollusks (51 species), and other phyla (23 species). Mean values of macrofaunal abundance were 1210, 2483, 3209, 3600 ind./m2 while those of biomass were 56.88, 176.15, 136.28, 265.55 g/m2, respectively. Ranges of species richness index, evenness index, Shannon-Wiener index were 1.54–8.16, 0.17–0.90, 0.69–4.78, respectively. The diversity indices were affected by bottom water salinity and pH. BIOENV analysis showed that water depth, phaeophorbide, and silt-clay proportion had important impacts on macrofaunal assemblages while abundance and biomass were affected by bottom water temperature, salinity, and water depth. Compared with historical data, macrofaunal species number, abundance and biomass showed a slight increasing trend, which may be due to the water quality improvement of this bay.

Polycyclic Aromatic Hydrocarbons Have Adverse Effects on Benthic Communities in the Baltic Sea: Implications for Environmental Status Assessment

Changes in benthic macrofaunal communities are indicative of environmental stressors, including eutrophication and hypoxia. However, some species are sensitive not only to hypoxia but also to various environmental contaminants. We tested which of the environmental predictors (sediment organic carbon, sediment concentrations of metals and polyaromatic hydrocarbons [PAHs], bottom water oxygen, salinity, temperature, and surface chlorophyll-a concentration) that best explained the following response variables: (1) macrofauna community composition, (2) abundance of a benthic sentinel species, the amphipod Monoporeia affinis; and (3) the Benthic Quality Index (BQI). All data originated from 29 reference monitoring stations in the Baltic Sea and the statistical tests included both uni- and multivariate analyses. The community composition and BQI were best explained by the same combination of salinity, depth, temperature and PAH concentrations. The abundance of M. affinis, which is sensitive to hypoxia and chemical exposure, was best explained by PAHs as a single predictor. Our findings suggest that benthic communities in the Baltic Sea are influenced by anthropogenic contaminants, which should be taken into account when benthos is used for eutrophication status assessment.

Spectral Analysis of Vertical Temperature Profile Time‐Series Data in Yellowstone Lake Sediments

AbstractWe use yearlong vertical temperature profile time‐series (seven thermistors at evenly spaced depth intervals from 10 to 70 cm) from five sites in and around the Deep Hole thermal area, southeast of Stevenson Island, Yellowstone Lake, to investigate heat and mass fluxes across the lake floor. The records demonstrate that thermal gradients in surficial sediments are modulated by a rich spectrum of bottom water temperature variations generated by hydrodynamic processes, and that sites inside the thermal area also respond to hydrothermal variations. We develop and implement a new method for estimating the sediment effective thermal diffusivity and pore fluid vertical flow rate that exploits the full spectrum of observed temperature variations to generate the parameter estimates, uncertainties, and metrics to assess statistical significance. Sediments at sites outside thermal areas have gradients of ∼7.5°C/m, in situ thermal diffusivities of ∼1.6 × 10−7 m2/s consistent with highly porous (80–90%) siliceous sediments, and experience hypolentic flow in the upper ∼20 cm. Sites inside the Deep Hole thermal area exhibit considerable spatial and temporal variability, with gradients of 1–32°C/m, and higher thermal diffusivities of ∼2–12 × 10−7 m2/s, consistent with hydrothermal alteration of biogenic silica to clays, quartz, and pyrite. Upward pore fluid flow at these sites is observed across multiple depth intervals, with maximum values of ∼3 cm/day. The observed spatial and temporal variability within the thermal area is consistent with upward finger flow combined with short wavelength convection within the porous sediments above a steam reservoir.

標識放流による北海道オホーツク海沿岸における未成熟ミズダコの移動，成長，経験水温および漁場水温の季節変化

標識放流による北海道オホーツク海沿岸における未成熟ミズダコの移動，成長，経験水温および漁場水温の季節変化

Millennial‐Scale Changes in Bottom Water Temperature and Water Mass Exchange Through the Fram Strait 79°N, 63‐13 ka

AbstractThe Svalbard margin, in the eastern Fram Strait with its high sediment accumulation, form a key area for the reconstruction of water mass and heat exchange between the North Atlantic and Arctic Ocean in relation to abrupt climate changes as seen in glacial Greenland Interstadial and Greenland Stadial (GI‐GS) events. Here, we present a bottom water temperature (BWT) record from the northern Nordic Seas (79°N) at 1,273 m water depth based on benthic foraminiferal Mg/Ca. The BWT reconstructions, combined with benthic foraminiferal stable isotopes, benthic foraminiferal fauna compositions and ice‐rafted debris (IRD), reveal at least two distinctive scenarios for the GI‐GS events during the last glacial period (13–63 ka). During GIs, conditions were similar to modern with high productivity, low BWT and deep convection. During GS6, GS8, and GS15 and during Heinrich Stadials (HSs), BWT increased up to 5°C ± 1°C generally concomitant with low planktic and benthic δ18O. Our results suggest, that during some GSs and HSs, deep water generation was reduced, allowing the subsurface Atlantic water (AW) to thicken and deepen down to at least the core site depth. A strong halocline during HSs and GSs prevented heat release from the subsurface AW, which we can now trace from 45°N in the North Atlantic to the Arctic Ocean >79°N. Surfacing of the salty Atlantic subsurface water preconditioned the Nordic seas for convection. Release of the subsurface heat from this vast reservoir must have contributed to the large and abrupt atmospheric warmings at the start of GIs.

Variance of Bottom Water Temperature at the Continental Margin of the Northern South China Sea

AbstractHigh‐resolution temperature variation was examined about 40 days near the ocean bottom at 20 sites within an area of 110 × 120 km2 close to the continental margin of the northern South China Sea (depth ≈ 4,000 m). The monitoring depth (0.5 m above the bottom) was presumably within the lower part of the bottom mixed layer, compared to its median thickness of 63 m. At all sites, the bottom water had temperature variance less than 0.01°C, and it alternatively varied in the near‐quiescent and internal wave states with time interval of diurnal period or even longer. Spectra of temperature variation showed that diurnal frequency dominated over the internal wave band. At different sites, the transition between internal wave band of scaling −2 and the inertial subrange turbulence of scaling −5/3 cannot easily be discriminated, attributed to the different intermittent presences of internal waves (tides). However, the resolved viscous‐convective and viscous‐diffusive subranges in the temperature spectrum provide the opportunity to evaluate the thermal (χ) and kinetic energy (ɛ) dissipation rates near the ocean bottom at each site. Close to a seamount, χ and ɛ were inferred to be about 1.2 × 10−12 °C2s−1 and 7.2 × 10−10 m2s−3, respectively, and they were strongly modulated by the diurnal internal tides. In the flat abyssal plain, χ and ɛ were much weaker, about at 3.2 × 10−14 °C2s−1 and 1.7 × 10−11 m2s−3, respectively. The spatial distribution of the nominal thermal χ, kinetic energy ɛ and the turbulent diffusivity Kρ indicated that they varied in the ranges [3.9 × 10−15 1.9 × 10−12] °C2s−1, [2.0 × 10−12 1.1 × 10−9] m2s−3, and [8.3 × 10−6 5.2 × 10−3] m2s−1, respectively, representing the different influences of internal waves and local topography.