Hydrothermal impacts on trace element and isotope ocean biogeochemistry German, C. R.; Casciotti, K. A.; Dutay, J.-C. ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
11/2016, Letnik:
374, Številka:
2081
Journal Article
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Hydrothermal activity occurs in all ocean basins, releasing high concentrations of key trace elements and isotopes (TEIs) into the oceans. Importantly, the calculated rate of entrainment of the ...entire ocean volume through turbulently mixing buoyant hydrothermal plumes is so vigorous as to be comparable to that of deep-ocean thermohaline circulation. Consequently, biogeochemical processes active within deep-ocean hydrothermal plumes have long been known to have the potential to impact global-scale biogeochemical cycles. More recently, new results from GEOTRACES have revealed that plumes rich in dissolved Fe, an important micronutrient that is limiting to productivity in some areas, are widespread above mid-ocean ridges and extend out into the deep-ocean interior. While Fe is only one element among the full suite of TEIs of interest to GEOTRACES, these preliminary results are important because they illustrate how inputs from seafloor venting might impact the global biogeochemical budgets of many other TEIs. To determine the global impact of seafloor venting, however, requires two key questions to be addressed: (i) What processes are active close to vent sites that regulate the initial high-temperature hydrothermal fluxes for the full suite of TEIs that are dispersed through non-buoyant hydrothermal plumes? (ii) How do those processes vary, globally, in response to changing geologic settings at the seafloor and/or the geochemistry of the overlying ocean water? In this paper, we review key findings from recent work in this realm, highlight a series of key hypotheses arising from that research and propose a series of new GEOTRACES modelling, section and process studies that could be implemented, nationally and internationally, to address these issues.
This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.
The Central American Seaway played a pivotal role in shaping global climate throughout the late Cenozoic. Recent geological surveys have provided new constraints on timing of the seaway shoaling, ...while neodymium isotopic (εNd) data measured on fossil teeth, debris, and ferromanganese crusts have helped define the history of water masses in the region. Here we provide the first 3‐D simulations of εNd responses to the shoaling seaway. Our model suggests that a narrow and shallow seaway is sufficient to affect interoceanic circulation, that inflow/outflow balance between the Caribbean and the Antilles responds nonlinearly to sill depth, and that a seaway narrower than 400 km is consistent with an active Atlantic meridional overturning circulation during the late Miocene. Simulated εNd values in the Caribbean confirm that inputs from radiogenic Pacific waters in the Caribbean decrease as the seaway shoals. Despite model limitations, a comparison between our results and εNd values recorded in the Caribbean helps constrain the depth of the Central American Seaway through time, and we infer that a depth between 50 and 200 m could have been reached 10 Ma ago.
Key Points
Model/data comparison of epsilon Nd suggests a shallow CAS during the Miocene
CAS throughflow depends on seaway geometry and atmospheric coupling
Ocean circulation responds nonlinearly to CAS shoaling
Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon‐cycle Model ...Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse‐resolution ocean circulation models. The model mean (±1σ) downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr−1. Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation‐based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep‐ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep‐ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross‐model means of several biogeochemical metrics show better agreement with observation‐based estimates when restricted to those models that best simulate deep‐ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics.
The decoupled behaviour observed between Nd isotopic composition (Nd IC, also referred as εNd) and Nd concentration cycles has led to the notion of a "Nd paradox". While εNd behaves in a ...quasi-conservative way in the open ocean, leading to its broad use as a water-mass tracer, Nd concentration displays vertical profiles that increase with depth, together with a deep-water enrichment along the global thermohaline circulation. This non-conservative behaviour is typical of nutrients affected by scavenging in surface waters and remineralisation at depth. In addition, recent studies suggest the only way to reconcile both concentration and Nd IC oceanic budgets, is to invoke a "Boundary Exchange" process (BE, defined as the co-occurrence of transfer of elements from the margin to the sea with removal of elements from the sea by Boundary Scavenging) as a source-sink term. However, these studies do not simulate the input/output fluxes of Nd to the ocean, and therefore prevents from crucial information that limits our understanding of Nd decoupling. To investigate this paradox on a global scale, this study uses for the first time a fully prognostic coupled dynamical/biogeochemical model with an explicit representation of Nd sources and sinks to simulate the Nd oceanic cycle. Sources considered include dissolved river fluxes, atmospheric dusts and margin sediment re-dissolution. Sinks are scavenging by settling particles. This model simulates the global features of the Nd oceanic cycle well, and produces a realistic distribution of Nd concentration (correct order of magnitude, increase with depth and along the conveyor belt, 65% of the simulated values fit in the ±10 pmol/kg envelop when compared to the data) and isotopic composition (inter-basin gradient, characterization of the main water-masses, more than 70% of the simulated values fit in the ±3 εNd envelop when compared to the data), though a slight overestimation of Nd concentrations in the deep Pacific Ocean may reveal an underestimation of the particle fields by the biogeochemical model. Our results indicate 1) vertical cycling (scavenging/remineralisation) is absolutely necessary to simulate both concentration and εNd, and 2) BE is the dominant Nd source to the ocean. The estimated BE flux (1.1×1010 g(Nd)/yr) is much higher than both dissolved river discharge (2.6×108 g(Nd)/yr) and atmospheric inputs (1.0×108 g(Nd)/yr) that both play negligible role in the water column but are necessary to reconcile Nd IC in surface and subsurface waters. This leads to a new calculated residence time of 360 yrs for Nd in the ocean. The BE flux requires the dissolution of 3 to 5% of the annual flux of continental weathering deposited via the solid river discharge to the continental margin.
The Mediterranean Sea is one of the most oligotrophic regions of the oceans, and nutrients have been shown to limit both phytoplankton and bacterial activities, resulting in a potential major role of ...dissolved organic carbon (DOC) export in the biological pump. Strong DOC accumulation in surface waters is already well documented, though measurements of DOC stocks and export flux are still sparse and associated with major uncertainties. This study provides the first basin-scale overview and analysis of organic carbon stocks and export fluxes in the Mediterranean Sea through a modeling approach based on a coupled model combining a mechanistic biogeochemical model (Eco3M-MED) and a high-resolution (eddy-resolving) hydrodynamic simulation (NEMO-MED12). The model is shown to reproduce the main spatial and seasonal biogeochemical characteristics of the Mediterranean Sea. Model estimations of carbon export are also of the same order of magnitude as estimations from in situ observations, and their respective spatial patterns are mutually consistent. Strong differences between the western and eastern basins are evidenced by the model for organic carbon export. Though less oligotrophic than the eastern basin, the western basin only supports 39 % of organic carbon (particulate and dissolved) export. Another major result is that except for the Alboran Sea, the DOC contribution to organic carbon export is higher than that of particulate organic carbon (POC) throughout the Mediterranean Sea, especially in the eastern basin. This paper also investigates the seasonality of DOC and POC exports as well as the differences in the processes involved in DOC and POC exports in light of intracellular quotas. Finally, according to the model, strong phosphate limitation of both bacteria and phytoplankton growth is one of the main drivers of DOC accumulation and therefore of export.
Sapropels are sediments rich in black, pelagic organic matter which occur mainly in the Eastern Mediterranean, documenting anoxic environments and high biological productivity. The quasiperiodicity ...of deposition of sapropels ‐over millions of years‐ relates to the Earth's precession cycle, which directly enhances the African monsoon, ultimately increasing freshwater input from the Nile. The last sapropel event, S1, occurred about 10,000 years ago (Early Holocene), when the Mediterranean region was warmer and wetter than today. Several modeling studies reflect the impact of this climate and a stronger Nile influx on Mediterranean oceanic circulation, but the regional models used lacked the spatial resolution necessary to simulate winter intermediate and deep convection. Here, we investigate recently occurring changes in the convective areas in the Eastern Mediterranean, using a regional ocean–atmosphere coupled climate model of high spatial resolution, essential to the simulation of a realistic Mediterranean circulation for present‐day conditions. We focused on the thermohaline circulation and the simulation of neodymium isotopic composition to compare our modeling results to modern data and paleo‐proxies. A sensitivity experiment shows a radical response of the Mediterranean to enhanced Nile discharge, creating the appropriate conditions for sapropel formation. We thus demonstrate that increased discharge of the Nile River can trigger the shutdown of Eastern Mediterranean convection and create conditions favorable to the development of anoxic events.
Key Points
High‐resolution numerical modeling of environment of deposition of sapropel S1
Nile river outflow enhancement triggers important changes in oceanic circulation
Boundary exchanges εNd modeling depicts reduced circulation, and records for S1 show a similar anomaly
Constraints on the Mediterranean Sea's storage of anthropogenic CO 2 are limited, coming only from data-based approaches that disagree by more than a factor of two. Here we simulate this marginal ...sea's anthropogenic carbon storage by applying a perturbation approach in a high-resolution regional model. Our model simulates that, between 1800 and 2001, basin-wide CO 2 storage by the Mediterranean Sea has increased by 1.0 Pg C, a lower limit based on the model's weak deep-water ventilation, as revealed by evaluation with CFC-12. Furthermore, by testing a data-based approach (transit time distribution) in our model, comparing simulated anthropogenic CO 2 to values computed from simulated CFC-12 and physical variables, we conclude that the associated basin-wide storage of 1.7 Pg, published previously, must be an upper bound. Out of the total simulated storage of 1.0 Pg C, 75 % comes from the air-sea flux into the Mediterranean Sea and 25 % comes from net transport from the Atlantic across the Strait of Gibraltar. Sensitivity tests indicate that the Mediterranean Sea's higher total alkalinity, relative to the global-ocean mean, enhances the Mediterranean's total inventory of anthropogenic carbon by 10 %. Yet the corresponding average anthropogenic change in surface pH does not differ significantly from the globalocean average, despite higher total alkalinity. In Mediterranean deep waters, the pH change is estimated to be between −0.005 and −0.06 pH units.
An extensive compilation of published Nd isotopic values has been made in order to establish a database and a map of the isotopic composition of Nd of the world coasts. Both the database and the map ...are set out here, together with the way we interpolated the data to make the map. The margin Nd isotopic signatures vary from non-radiogenic values around the Atlantic Ocean to radiogenic values around the Pacific consistent with the trajectory of the “conveyor belt” global circulation, reinforcing the hypothesis that the exchange of Nd along the margins could play a significant role in driving the oceanic distribution of this tracer.
The semi-enclosed nature of the Mediterranean Sea, together with its smaller inertia due to the relative short residence time of its water masses, make it highly reactive to external forcings, in ...particular variations of water, energy and matter fluxes at the interfaces. This region, which has been identified as a “hotspot” for climate change, is therefore expected to experience environmental impacts that are considerably greater than those in many other places around the world. These natural pressures interact with the increasing demographic and economic developments occurring heterogeneously in the coastal zone, making the Mediterranean even more sensitive. This review paper aims to provide a review of the state of current functioning and responses of Mediterranean marine biogeochemical cycles and ecosystems with respect to key natural and anthropogenic drivers and to consider the ecosystems’ responses to likely changes in physical, chemical and socio-economical forcings induced by global change and by growing anthropogenic pressure at the regional scale. The current knowledge on and expected changes due to single forcing (hydrodynamics, solar radiation, temperature and acidification, chemical contaminants) and combined forcing (nutrient sources and stoichiometry, extreme events) affecting the biogeochemical fluxes and ecosystem functioning are explored. Expected changes in biodiversity resulting from the combined action of the different forcings are proposed. Finally, modeling capabilities and necessity for modeling are presented. A synthesis of our current knowledge of expected changes is proposed, highlighting relevant questions for the future of the Mediterranean ecosystems that are current research priorities for the scientific community. Finally, we discuss how these priorities can be approached by national and international multi-disciplinary research, which should be implemented on several levels, including observational studies and modeling at different temporal and spatial scales.
The Mediterranean region is a climate change
hotspot. Increasing greenhouse gas emissions are projected to lead to a
substantial warming of the Mediterranean Sea as well as major changes in its
...circulation, but the subsequent effects of such changes on marine
biogeochemistry are poorly understood. Here, our aim is to investigate how
climate change will affect nutrient concentrations and biological
productivity in the Mediterranean Sea. To do so, we perform transient
simulations with the coupled high-resolution model NEMOMED8-PISCES using the
high-emission IPCC Special Report on Emissions
Scenarios (SRES) A2 socioeconomic scenario and corresponding
Atlantic, Black Sea, and riverine nutrient inputs. Our results indicate that
nitrate is accumulating in the Mediterranean Sea over the 21st century, while
phosphorus shows no tendency. These contrasting changes result from an
unbalanced nitrogen-to-phosphorus input from riverine discharge and fluxes
via the Strait of Gibraltar, which lead to an expansion of phosphorus-limited
regions across the Mediterranean. In addition, phytoplankton net primary
productivity is reduced by 10 % in the 2090s in comparison to the present
state, with reductions of up to 50 % in some regions such as the Aegean
Sea as a result of nutrient limitation and vertical stratification. We also
perform sensitivity tests to separately study the effects of climate and
biogeochemical input changes on the future state of the Mediterranean Sea.
Our results show that changes in nutrient supply from the Strait of Gibraltar
and from rivers and circulation changes linked to climate change may have
antagonistic or synergistic effects on nutrient concentrations and surface
primary productivity. In some regions such as the Adriatic Sea, half of the
biogeochemical changes simulated during the 21st century are linked with
external changes in nutrient input, while the other half are linked to climate
change. This study is the first to simulate future transient climate change
effects on Mediterranean Sea biogeochemistry but calls for further work to
characterize effects from atmospheric deposition and to assess the various
sources of uncertainty.
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