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Sea-level variations influence weak layer formation and submarine landslides on a low-latitude continental margin
No full textWeak layers have been identified as lithological contrasts on continental slopes to precondition giant submarine landslides. Although sea-level variations are well known to generate weak layers on glaciated continental margins at high latitudes, their role on non-glaciated continental slopes, mostly at low latitudes, remains poorly constrained. Here, we combine seismic and well-log data, showing that the glide plane beneath the giant Baiyun Slide Complex in the South China Sea is located at a lithological interface, within a relatively coarse-grained unit that was deposited during the late Pliocene sea-level drop. Fluid migration pathways are further identified in the underlying fine-grained unit, which was deposited during the early Pliocene sea-level rise. Sea-level variations are therefore proposed to form unstable sedimentary sequences by the deposition of interbedded fine- and coarse-grained sediments. Considering the global records of sea-level variations and consequent lithological contrasts on non-glaciated slopes, our results shed light on the development of slope instability in low-latitude oceans
Comparison of picolyl azide-based BONCAT and microautoradiography for assessing the heterotrophic prokaryotic activity in the deep ocean
No full textProkaryotes play a central role in marine biogeochemical cycles, yet quantifying their activity requires sensitive methods due to low biomass and metabolic rates, particularly in the deep ocean. One recent method to determine single-cell activity of prokaryotes is bioorthogonal non-canonical amino acid tagging (BONCAT), which offers a non-radioactive approach to measure protein synthesis. However, direct comparisons between BONCAT and radioisotope-based techniques across ocean depth gradients remain limited, particularly for low-activity prokaryotic communities. To address this knowledge gap, we tested an optimised BONCAT protocol using picolyl azide fluorophores (BONCAT-pic) to assess single-cell heterotrophic activity in prokaryotic communities from surface to bathypelagic depths (1000–4000 m) in the Southern Ocean near the Kerguelen Islands. The method was first optimised using aged coastal and open-ocean seawater, and then compared to microautoradiography with 3H-methionine uptake. Statistical analysis shows that BONCAT-pic significantly improved detection sensitivity compared to standard azide reagents. BONCAT-pic consistently detected active cells in profiles over the open ocean water column, with cell proportions and fluorescence signals closely correlating with both microautoradiography (R2 = 0.9, p < 0.001) and bulk methionine incorporation (R2 = 0.6, p < 0.001). Our results demonstrate that BONCAT-pic is a reliable, fluorescence-based method for quantifying heterotrophic activity at the single-cell level, extending its applicability to prokaryotic communities in the deep ocean
Consistent prokaryotic successional dynamics across contrasting phytoplankton blooms
No full textHeterotrophic prokaryotes play a vital role in organic matter cycling in the ocean and have been observed to undergo substrate‐controlled successions during phytoplankton blooms. However, there is limited understanding of the succession patterns during blooms triggered by upwelling events of different characteristics. Here we simulated eight upwelling scenarios of varying intensity and duration (single vs. recurring pulses) by adding nutrient‐rich mesopelagic waters into large‐scale mesocosms containing oligotrophic surface waters from the subtropical North Atlantic. Over a monitoring period of nearly 6 weeks, we observed that phytoplankton blooms displayed diverging outcomes depending on the upwelling mode: treatments with single upwelling pulses presented a unique, short‐lived bloom, whereas recurring upwelling resulted in blooms that were sustained over time. Prokaryotic abundances were positively related to upwelling intensity and presented three similar abundance cycles in all treatments, whereas heterotrophic activity differed between the two upwelling modes. The successional dynamics of free‐living and particle‐associated communities were consistent regardless of upwelling intensity and mode, with four or five prokaryotic assemblages sequentially proliferating during the experiment. Yet, some differences were observed in the taxa that formed the assemblages in both upwelling modes. Together, our results suggest that, despite differences in activity, prokaryotes seemed to be more influenced by processes taking place within the community than by phytoplankton bloom patterns, with similar succession dynamics even under widely distinct blooms. These findings can help advance our understanding on prokaryotic ecology and its relation to organic matter cycling across different upwelling scenarios
Dynamics of Near‐Bottom Currents in Cold‐Water Coral and Sponge Areas at Valdivia Bank and Ewing Seamount, Southeast Atlantic
No full textThis study investigates near‐bottom currents and physical processes from simulations with the hydrodynamic model ROMS‐AGRIF at two seamounts of the northeast Walvis Ridge to obtain valuable insights about drivers of observed occurrences of benthic suspension feeders (cnidarians and sponges) in this data‐poor area. The spatial resolution in each model area was increased across two levels of nested grids from 1,500 m to 500 m resolution with 32 stretched terrain‐following (s‐) layers in the vertical with high resolution close to the bottom. The parent grids receive initial and boundary conditions from the basin‐scale model INALT20 and from solutions of the OTIS inverse tidal model. The model topography is based on GEBCO data with local refinements from multi‐beam data collected during different surveys in 2008, 2009, and 2010. Increasing model resolution is an important advancement for precisely evaluating the intrinsic dynamics within challenging rough terrain. We examined how near‐bottom currents vary over space and time and investigated potential links between observed Cnidarian and Porifera occurrences and ranges of physical variables and processes. We identified a close link between physical processes and species distributions and suggested that physical processes such as kinetic energy dissipation and internal wave dynamics may be considered in future research as proxies of food supply to benthic suspension feeders. Such mechanistic variables may also be used to supplement more traditional descriptors such as water mass and terrain properties in species distribution models, thus enhancing our ability to predict the occurrence of benthic communities characterized by cnidarians and sponges.
Plain Language Summary
We investigated the physical drivers of the distribution of cnidarians and sponges in two areas along the northeast Walvis Ridge, Valdivia Bank, and Ewing Seamount based on data from ocean model simulations over a period of 3 years. To get realistic boundary conditions for our simulations, we added bathymetric data from high‐resolution seafloor topography, water column data from a larger‐scale ocean circulation model, and information about tidal currents. By refining our model, we were able to better resolve the near‐bottom circulation and how it changes over time and space. Incorporating finer‐scale variability of physical variables and processes that represent important proxies for the transport and delivery of food to deep‐sea benthic fauna has the potential to greatly improve predictions of benthic filter feeding communities. Moreover, integration of such variables in future species distribution modeling may contribute to our current understanding of optimal environmental envelopes for important deep‐sea taxa, such as those that represent vulnerable marine ecosystems.
Key Points
High‐resolution nested hydrodynamic models simulate multiyear variations of bottom currents at two Walvis Ridge seamounts
Near‐bottom currents are influenced by flow‐topography interactions and internal tide dynamics
Kinetic energy dissipation and internal tide dynamics are important mechanisms driving food supply to benthic suspension feeder
Climate change risks on key open marine andcoastal Mediterranean ecosystems
No full textMediterranean open marine and coastal ecosystems face multiple risks, due to climate change, that impact their unique biodiversity. To assess these risks and evaluate their confidence levels, we adopt the scenario-based approach of the Intergovernmental Panel on Climate Change (IPCC), relying on a review of literature projecting changes in Mediterranean Sea ecosystems. The main drivers of environmental change are sea level rise, ocean warming and acidification. Similar to global conditions, all Mediterranean ecosystems face high risks under all climate scenarios, with coastal ecosystems being more strongly impacted than open marine ecosystems. For these coastal ecosystems, risk levels are expected to become very high already once global warming exceeds 0.8°C with respect to the 1976–2005 period. A few Mediterranean ecosystems (e.g., coralligenous and rocky coasts) have greater adaptive capacity than all others, probably because of the long evolutionary history in this sea and the presence of a variety of climatic and hydrological conditions. Overall, due to the higher observed and projected rates of climate change in the Mediterranean, compared to global trends, for variables such as seawater temperature and pH, marine ecosystems (particularly coastal) are projected to be under higher risks compared to the global ocean
A Close Look at Dissolved Silica Dynamics in Disko Bay, West Greenland
No full textDischarge of calved ice, runoff and mixing driven by subglacial discharge plumes likely have consequences for marine biogeochemistry in Disko Bay, which hosts the largest glacier in the northern hemisphere, Sermeq Kujalleq. Glacier retreat and increasing runoff may impact the marine silica cycle because glaciers deliver elevated concentrations of dissolved silica (dSi) compared to other macronutrients. However, the annual flux of dSi delivered to the ocean from the Greenland Ice Sheet is poorly constrained because of difficulties distinguishing the overlapping influence of different dSi sources. Here we constrain silica dynamics around Disko Bay, including the Ilulissat Icefjord and four other regions receiving glacier runoff with contrasting levels of productivity and turbidity. Both dissolved silica and Si* ([dSi]‐[NO x − ]) concentrations indicated conservative dynamics in two fjords with runoff from land‐terminating glaciers, consistent with the results of mixing experiments. In three fjords with marine‐terminating glaciers, macronutrient‐salinity distributions were strongly affected by entrainment of nutrients in subglacial discharge plumes. Entrainment of dSi from saline waters explained 93 ± 51% of the dSi enrichment in the outflowing plume from Ilulissat Icefjord, whereas the direct contribution of freshwater to dSi in the plume was likely 0%–3%. Whilst not distinguished herein, other minor regional dSi sources include icebergs and dissolution of amorphous silica (aSi) in either pelagic or benthic environments. Our results suggest that runoff around Greenland is supplemented as a dSi source by minor fluxes of 0.25 ± 0.67 Gmol yr −1 dSi from icebergs and ∼1.9 Gmol year −1 from pelagic aSi dissolution.
Plain Language Summary
Silica is one nutrient required by marine phytoplankton, specifically siliceous microalgae such as diatoms. Glacier runoff delivers higher concentrations of silica into the ocean compared to other nutrients such as nitrate or phosphate. Changes in the cryosphere, such as glacier retreat and increasing ice discharge or runoff, may therefore have downstream ecological effects due to shifts in the availability, and ratios, of nutrients. However, the magnitude of dissolved silica fluxes into the ocean from the present day Greenland Ice Sheet has proven challenging to determine, with two existing estimates varying by an order of magnitude. This is because of uncertainties in how to disentangle the overlapping influence of different dSi sources. Here, we conducted a detailed survey of the Disko Bay region in west Greenland, including inshore and offshore dSi measurements, to assess how the chemistry of dSi changes in estuaries. In order to further understand estuarine dSi dynamics, we conducted incubation experiments at sea and in the laboratory. Our results reduce uncertainty in the magnitude of dSi supplied to the ocean from Greenland's glaciers and suggest a modest dSi flux from runoff with small additional fluxes from melting icebergs and dSi release from suspended particles.
Key Points
Both runoff and entrainment in subglacial discharge plumes influence estuarine nutrient dynamics around Disko Bay
Entrainment from saline waters explains 93 ± 51% of dSi enrichment in the Ilulissat Icefjord outflow, freshwater <3%
We estimate 0.25 and ∼1.9 Gmol yr −1 dSi fluxes from Greenland's icebergs and pelagic aSi dissolution, respectivel
Widespread occurrence of phosphate storage in foraminifera: Adaption to O2 depletion and relevance for P-cycling
No full textForaminifera are ubiquitous marine protists that intracellularly accumulate phosphate1, an important macronutrient in marine ecosystems and in fertilizer potentially leaked into the ocean. Intracellular phosphate concentrations can be 100-1,000 times higher than in the surrounding water1. Here we show that phosphate storage in foraminifera is widespread, from tidal flats to the deep sea. The total amount of intracellular phosphate stored in the benthic foraminifer Ammonia confertitesta in the Wadden Sea during a bloom is as high as around 5% of the annual consumption of phosphorus (P) fertilizer in Germany. Budget calculations for the Southern North Sea and the Peruvian Oxygen Minimum Zone indicate that benthic foraminifera may buffer riverine P runoff for approximately 37 days at the Southern North Sea and for about 21 days at the Peruvian margin. This indicates that these organisms are probably relevant for marine P cycling-they potentially buffer anthropogenic eutrophication in coastal environments. Phosphate is stored as polyphosphate in cell organelles that are potentially acidocalcisomes. Their metabolic functions can range from regulation of osmotic pressure and intracellular pH to calcium and energy storage. In addition, storage of energetic P compounds, such as creatine phosphate and polyphosphate, is probably an adaptation of foraminifera to O2 depletion
Dynamics of marine inorganic carbon and silica: A field study of the mechanisms controlling seawater major element concentrations
No full textA highly resolved time series of dissolved major element (calcium, strontium, magnesium, and lithium) concentrations in the north Gulf of Aqaba, Red Sea, reveals variability in major cation concentrations beyond analytic uncertainties. This variability is composed of an interannual component that is most important for calcium, and a short-term daily-timescale component that is most important for lithium. As evident from covariation in calcium, potential alkalinity, and Sr/Ca, the calcium carbonate cycle of the Gulf of Aqaba is dominated by coral calcification, and there was an increase in calcification rates between 2017 and 2018. Variability in lithium concentrations, and larger changes in magnesium concentrations than expected from magnesium distribution coefficients in carbonate minerals, suggest an active cycle of aluminosilicate mineral dissolution, and precipitation of secondary silicate minerals
The effect of calcium supplementation on bone calcium balance and calcium and bone metabolism during load carriage in women: a randomised controlled crossover trial
No full textCalcium supplementation before exercise attenuates the decrease in serum calcium and increase in PTH and bone resorption. This study investigated the effect of calcium supplementation on calcium and bone metabolism during load carriage in women. Forty-eight women completed two load carriage sessions (load carriage 1 n = 48; load carriage 2 n = 40) (12.8 km in 120 min carrying 20 kg) 60 min after consuming either 1000 mg calcium (Calcium) or nothing (Control) in a randomised order. Pre- and post-exercise urine samples were analysed for calcium isotope ratio (δ44/42Ca). Fasted blood samples were taken before (pre-exercise), during (0, 20, 40, 60, 80, 100, 120 min), and after (+15, +30, +60, +90 min) exercise and analysed for markers of calcium and bone metabolism. There was no effect of load carriage or supplementation on urine δ44/42Ca (P≥.110). Serum δ44/42Ca did not change with load carriage in Control (P=.617) but increased in Calcium (P=.003) and was higher at 120 min in Calcium vs Control (P=.018). Ionised calcium (iCa) decreased from pre-exercise to all exercise time-points (P<.001); iCa was higher in Calcium than Control throughout (P<.001). PTH increased from pre-exercise to 120 min in Control (P<.001) but decreased from pre-exercise to all time-points in Calcium (P<.001). PTH was higher in Control than Calcium from 0 to +90 min (P<.001). βCTX decreased from pre-exercise to 20 to +15 min in Control (P≤.004); βCTX decreased from pre-exercise to 0 to +90 min in Calcium (P<.001). βCTX was lower in Calcium than Control from 20 to +90 min (P≤.036). A 1000 mg calcium supplement before load carriage promotes bone calcium balance and prevents disruptions to bone and calcium homeostasis
Seismic Structure and Tectonics of the North‐Central Chilean Subduction Zone Along the Copiapó Ridge From Amphibious Seismic Refraction Tomography and Local Seismicity
No full textThe Chilean margin is one of the Earth's tectonically most active plate boundaries, and yet, some of its segments are still underexplored. Here, we present amphibious data from the Copiapó region at ∼27°S located within the mature Atacama seismic gap. Combined 2D seismic refraction, multibeam bathymetry, and local seismicity data show a typical oceanic crust thickness of 6–7 km and seismic P‐wave velocities between 3.0 and 7.3 km/s with slightly lower velocities and increased thicknesses underneath the Copiapó Ridge seamounts. The latter is most likely due to predominantly extrusive formation. Elevated velocities underneath one of the seamounts indicate a local region of magmatic underplating, while bending‐related faults visible in the bathymetry and reduced mantle velocities near the trench suggest mantle hydration. The subduction angle of the down‐going Nazca plate smoothly increases from 12° below the marine forearc to 22° at greater depths (40–60 km) with no abrupt change in the dip angle as observed at ∼22°S. The local seismicity off‐ and onshore Copiapó shows three separated bands of earthquakes sub‐parallel to the down‐going plate, and are most likely related to the plate interface, the oceanic Moho and the Double Benioff Zone. The largest event (M W 5.9) during our observation period (December 2022–June 2023) and its aftershocks occurred in the deepest band ∼20 km below the subduction interface. Along the interface, seismicity is most pronounced in areas of high locking offshore, whereas areas of low locking are characterized by previously observed slow slip events and sparse seismicity.
Plain Language Summary
The Chilean margin is characterized by the collision and subduction of the oceanic Nazca Plate below the continental South American Plate, repeatedly causing large earthquakes. In this study, we investigate the region around Copiapó (∼27°S), which has not been affected by a megathrust earthquake for more than 100 years. By combining different amphibious methods (high‐resolution multibeam bathymetry, wide‐angle seismic refraction and seismological data), we conclude that the Copiapó Ridge seamounts were formed predominantly by extrusive volcanic processes and that the crust underneath some seamounts is thickened due to ascending magma. Compared to ∼22°S, where the subduction angle shows an abrupt steepening, we observe a smooth increase from 12° at shallow depths to 22° at greater depths. Furthermore, our analysis reveals that earthquakes occur in three bands at different depths that are sub‐parallel to the subducting plate. The largest earthquake and its aftershocks occurred in the deepest band and were not related to the interface between the colliding plates.
Key Points
Multidisciplinary approach using high‐resolution bathymetry, amphibious refraction tomography and shoreline‐crossing seismicity
P‐wave velocity model reveals seamounts formed by extrusive volcanism with possible underplating added underneath typical oceanic crust
Seismicity occurs in three bands sub‐parallel to the plate interface, with the largest event and aftershocks located ∼20 km below the interfac