Posts Tagged 'globalmodeling'

An alternative to static climatologies: robust estimation of open ocean CO2 variables and nutrient concentrations from T, S, and O2 data using Bayesian neural networks

This work presents two new methods to estimate oceanic alkalinity (AT), dissolved inorganic carbon (CT), pH, and pCO2 from temperature, salinity, oxygen, and geolocation data. “CANYON-B” is a Bayesian neural network mapping that accurately reproduces GLODAPv2 bottle data and the biogeochemical relations contained therein. “CONTENT” combines and refines the four carbonate system variables to be consistent with carbonate chemistry. Both methods come with a robust uncertainty estimate that incorporates information from the local conditions. They are validated against independent GO-SHIP bottle and sensor data, and compare favorably to other state-of-the-art mapping methods. As “dynamic climatologies” they show comparable performance to classical climatologies on large scales but a much better representation on smaller scales (40–120 d, 500–1,500 km) compared to in situ data. The limits of these mappings are explored with pCO2 estimation in surface waters, i.e., at the edge of the domain with high intrinsic variability. In highly productive areas, there is a tendency for pCO2 overestimation due to decoupling of the O2 and C cycles by air-sea gas exchange, but global surface pCO2 estimates are unbiased compared to a monthly climatology. CANYON-B and CONTENT are highly useful as transfer functions between components of the ocean observing system (GO-SHIP repeat hydrography, BGC-Argo, underway observations) and permit the synergistic use of these highly complementary systems, both in spatial/temporal coverage and number of observations. Through easily and robotically-accessible observations they allow densification of more difficult-to-observe variables (e.g., 15 times denser AT and CT compared to direct measurements). At the same time, they give access to the complete carbonate system. This potential is demonstrated by an observation-based global analysis of the Revelle buffer factor, which shows a significant, high latitude-intensified increase between +0.1 and +0.4 units per decade. This shows the utility that such transfer functions with realistic uncertainty estimates provide to ocean biogeochemistry and global climate change research. In addition, CANYON-B provides robust and accurate estimates of nitrate, phosphate, and silicate. Matlab and R code are available at https://github.com/HCBScienceProducts/.

Continue reading ‘An alternative to static climatologies: robust estimation of open ocean CO2 variables and nutrient concentrations from T, S, and O2 data using Bayesian neural networks’

Strategies in times of crisis—insights into the benthic foraminiferal record of the Palaeocene–Eocene Thermal Maximum

Climate change is predicted to alter temperature, carbonate chemistry and oxygen availability in the oceans, which will affect individuals, populations and ecosystems. We use the fossil record of benthic foraminifers to assess developmental impacts in response to environmental changes during the Palaeocene–Eocene Thermal Maximum (PETM). Using an unprecedented number of µ-computed tomography scans, we determine the size of the proloculus (first chamber), the number of chambers and the final size of two benthic foraminiferal species which survived the extinction at sites 690 (Atlantic sector, Southern Ocean, palaeodepth 1900 m), 1210 (central equatorial Pacific, palaeodepth 2100 m) and 1135 (Indian Ocean sector, Southern Ocean, palaeodepth 600–1000 m). The population at the shallowest site, 1135, does not show a clear response to the PETM, whereas those at the other sites record reductions in diameter or proloculus size. Temperature was similar at all sites, thus it is not likely to be the reason for differences between sites. At site 1210, small size coincided with higher chamber numbers during the peak event, and may have been caused by a combination of low carbonate ion concentrations and low food supply. Dwarfing at site 690 occurred at lower chamber numbers, and may have been caused by decreasing carbonate saturation at sufficient food levels to reproduce. Proloculus size varied strongly between sites and through time, suggesting a large influence of environment on both microspheric and megalospheric forms without clear bimodality. The effect of the environmental changes during the PETM was more pronounced at deeper sites, possibly implicating carbonate saturation.

Continue reading ‘Strategies in times of crisis—insights into the benthic foraminiferal record of the Palaeocene–Eocene Thermal Maximum’

Drivers of future seasonal cycle changes in oceanic pCO2

Recent observation-based results show that the seasonal amplitude of surface ocean partial pressure of CO2 (pCO2) has been increasing on average at a rate of 2–3µatm per decade (Landschützer et al. 2018). Future increases in pCO2 seasonality are expected, as marine CO2 concentration ([CO2]) will increase in response to increasing anthropogenic carbon emissions (McNeil and Sasse 2016). Here we use seven different global coupled atmosphere–ocean–carbon cycle–ecosystem model simulations conducted as part of the Coupled Model Intercomparison Project Phase 5 (CMIP5) to study future projections of the pCO2 annual cycle amplitude and to elucidate the causes of its amplification. We find that for the RCP8.5 emission scenario the seasonal amplitude (climatological maximum minus minimum) of upper ocean pCO2 will increase by a factor of 1.5 to 3 over the next 60–80 years. To understand the drivers and mechanisms that control the pCO2 seasonal amplification we develop a complete analytical Taylor expansion of pCO2 seasonality in terms of its four drivers: dissolved inorganic carbon (DIC), total alkalinity (TA), temperature (T), and salinity (S). Using this linear approximation we show that the DIC and T terms are the dominant contributors to the total change in pCO2 seasonality. To first order, their future intensification can be traced back to a doubling of the annual mean pCO2, which enhances DIC and alters the ocean carbonate chemistry. Regional differences in the projected seasonal cycle amplitude are generated by spatially varying sensitivity terms. The subtropical and equatorial regions (40°S–40°N) will experience a  ≈ 30–80µatm increase in seasonal cycle amplitude almost exclusively due to a larger background CO2 concentration that amplifies the T seasonal effect on solubility. This mechanism is further reinforced by an overall increase in the seasonal cycle of T as a result of stronger ocean stratification and a projected shoaling of mean mixed layer depths. The Southern Ocean will experience a seasonal cycle amplification of  ≈ 90–120µatm in response to the mean pCO2-driven change in the mean DIC contribution and to a lesser extent to the T contribution. However, a decrease in the DIC seasonal cycle amplitude somewhat counteracts this regional amplification mechanism.

Continue reading ‘Drivers of future seasonal cycle changes in oceanic pCO2’

Lower Triassic deep sea carbonate precipitates from South Tibet, China

Sea-floor carbonate precipitates (SCPs), commonly seen in pre-Cambrian strata, were widely developed during the Permian–Triassic mass extinction and the Early Triassic recovery interval. Most SCPs are found in shallow water facies, with few SCPs reported from deep sea settings. Here, we document Lower Triassic deep sea SCPs from turbidite deposits exposed at the Xiukang section, South Tibet, China. The SCPs only occur within thin-bedded, silty limestones that are embedded in dark grey shale. Parallel–aligned mud cobbles, rounded micritic intraclasts and micro-erosional surfaces are commonly seen in these planar laminated limestones that contain abundant radiolarians and thin-shelled bivalves, indicating deposition in a deep basin via turbidity currents. The deep sea SCPs, which are comprised of bladed calcite crystals, display a weak vertical δ13C variation with the overlying matrix, and a uniform element distribution and consequently have a homogenous cathodoluminescence pattern, suggesting rapid precipitation in the open ocean, followed by a quick burial, which led to minimal diagenetic effects on the SCPs. We propose a new hypothesis that rapid carbonate precipitation resulted from mixing, driven by the turbidity current that introduced shallow seawater to the deep seawaters with a high alkalinity. Based on this model, the turbidity currents during the Early Triassic might serve as a potential role in the connection between the oxic surface oceans and the euxinic, deep oceans.

Continue reading ‘Lower Triassic deep sea carbonate precipitates from South Tibet, China’

Reverse weathering as a long-term stabilizer of marine pH and planetary climate

For the first four billion years of Earth’s history, climate was marked by apparent stability and warmth despite the Sun having lower luminosity1. Proposed mechanisms for maintaining an elevated partial pressure of carbon dioxide in the atmosphere pCO2 centre on a reduction in the weatherability of Earth’s crust and therefore in the efficiency of carbon dioxide removal from the atmosphere2. However, the effectiveness of these mechanisms remains debated2,3. Here we use a global carbon cycle model to explore the evolution of processes that govern marine pH, a factor that regulates the partitioning of carbon between the ocean and the atmosphere. We find that elevated rates of ‘reverse weathering’—that is, the consumption of alkalinity and generation of acidity during marine authigenic clay formation4,5,6,7—enhanced the retention of carbon within the ocean–atmosphere system, leading to an elevated pCO2 baseline. Although this process is dampened by sluggish kinetics today, we propose that more prolific rates of reverse weathering would have persisted under the pervasively silica-rich conditions8,9 that dominated Earth’s early oceans. This distinct ocean and coupled carbon–silicon cycle state would have successfully maintained the equable and ice-free environment that characterized most of the Precambrian period. Further, we propose that during this time, the establishment of a strong negative feedback between marine pH and authigenic clay formation would have also enhanced climate stability by mitigating large swings in pCO2—a critical component of Earth’s natural thermostat that would have been dominant for most of Earth’s history. We speculate that the late ecological rise of siliceous organisms8 and a resulting decline in silica-rich conditions dampened the reverse weathering buffer, destabilizing Earth’s climate system and lowering baseline pCO2.

Continue reading ‘Reverse weathering as a long-term stabilizer of marine pH and planetary climate’

The location and protection status of earth’s diminishing marine wilderness

Highlights

  • We classify 13.2% (∼55 million km2) of the world’s ocean as marine wilderness
  • Little wilderness remains in coastal areas (e.g., coral reefs)
  • Only 4.9% of marine wilderness is currently within marine protected areas
  • Targets to retain marine wilderness are needed in global conservation strategies

Summary

As human activities increasingly threaten biodiversity areas devoid of intense human impacts are vital refugia. These wilderness areas contain high genetic diversity, unique functional traits, and endemic species; maintain high levels of ecological and evolutionary connectivity; and may be well placed to resist and recover from the impacts of climate change. On land, rapid declines in wilderness.  Here we systematically map marine wilderness globally by identifying areas that have both very little impact (lowest 10%) from 15 anthropogenic stressors and also a very low combined cumulative impact from these stressors. We discover that ∼13% of the ocean meets this definition of global wilderness, with most being located in the high seas. Recognizing that human influence differs across ocean regions, we repeat the analysis within each of the 16 ocean realms Realm-specific wilderness extent varies considerably, with >16 million km2 (8.6%) in the Warm Indo-Pacific, down to <2,000 km2 (0.5%) in Temperate Southern Africa. We also show that the marine protected area estate holds only 4.9% of global wilderness and 4.1% of realm-specific wilderness, very little of which is in biodiverse ecosystems such as coral reefs. Proactive retention of marine wilderness should now be incorporated into global strategies aimed at conserving biodiversity and ensuring that large-scale ecological and evolutionary processes continue.

Continue reading ‘The location and protection status of earth’s diminishing marine wilderness’

Future ecosystem changes in the Northeast Atlantic: a comparison between a global and a regional model system

The biogeochemistry from a global climate model (Norwegian Earth System Model) has been compared with results from a regional model (NORWECOM.E2E), where the regional model is forced by downscaled physics from the global model. The study should both be regarded as a direct comparison between a regional and its driving global model to investigate at what extent a global climate model can be used for regional studies, and a study of the future climate change in the Nordic and Barents Seas. The study concludes that the global and regional model compare well on trends, but many details are lost when a coarse resolution global model is used to assess climate impact on regional scale. The main difference between the two models is the timing of the spring bloom, and a non-exhaustive nutrient consumption in the global model in summer. The global model has a cold (in summer) and saline bias compared with climatology. This is both due to poorly resolved physical processes and oversimplified ecosystem parameterization. Through the downscaling the regional model is to some extent able to alleviate the bias in the physical fields, and the timing of the spring bloom is close to observations. The summer nutrient minimum is one month early. There is no trend in future primary production in any of the models, and the trends in modelled pH and ΩAr are also the same in both models. The largest discrepancy in the future projection is in the development of the CO2 uptake, where the regional suggests a slightly reduced uptake in the future.

Continue reading ‘Future ecosystem changes in the Northeast Atlantic: a comparison between a global and a regional model system’


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