Ocean Biological Productivity Research Articles

Future climate change in the Northern Indian Ocean as simulated with a high-resolution regional earth system model

This study examines the future climate change in the South Asia region during 2070–2099 with respect to the historical period (1975–2004) under RCP8.5 scenario using a high-resolution regional earth system model. We found substantial changes in the key climatic parameters over the South Asia region including ocean biological productivity, however, the magnitude of response varies spatially. A substantial increase (> 2.5 °C) in the projected annual-mean sea surface temperature (SST) was found over the Indian Ocean with the highest increase (~ 3.4 °C) locally in the northern part of the Arabian Sea and in the Persian Gulf, SST changes being significant throughout the study area with 95% confidence level. The changes in the sea surface salinity showed strong spatial variability with the highest freshening over northern Bay of Bengal and highest salinity in the Persian Gulf followed by northern Arabian Sea. The amount of annual-mean precipitation will substantially increase over the eastern coast of the Bay of Bengal (up to 1.5–2.0 mm/day) and along the equator in the band 10° S–10° N (0.5–1.5 mm/day), while it will decrease over the western part of the Bay of Bengal and in the northern states of India (− 0.5 to 1.0 mm/day). The most pronounced increase of precipitation rate in the future climate will occur over India (3–5 mm/day) and the eastern coasts of the Bay of Bengal (> 5 mm/day) during the monsoon period, and over the equatorial band (2–3 mm/day) during the post-monsoon period, with all precipitation changes indicated above being significant at 95% confidence level.

Inorganic nitrogen and phosphorus in Western European aerosol and the significance of dry deposition flux into stratified shelf waters

Dry deposition of nitrogen (N) and phosphorus (P) from the aerosol phase represents a potential source of nutrients to marine surface waters. To investigate the significance of this deposition pathway, aerosol samples were collected from Penlee Point Atmospheric Observatory in SW England, UK, over a 6-month period (February to July 2015) covering the spring bloom. Samples were analysed for nitrate, ammonium and phosphate and the dry deposition flux of these nutrients calculated to assess its potential impact on primary production in nearby surface seawater. Aerosol-derived N and P deposition fluxes ranged from 2.7 to 620 μmol N m−2 d−1 and 0.16–1.6 μmol P m−2 d−1, respectively. Air mass back trajectory analysis indicated that the highest N fluxes were associated with polluted European air masses, highlighting a significant anthropogenic influence on N-content of aerosols. The N:P ratios of aerosol fluxes and water column concentrations indicated that P deposition was unlikely to be biologically significant in the region. In contrast, aerosol deposition was a significant episodic source of new N to marine phytoplankton after the onset of water column stratification. Carbon fixation estimates indicated that the maximum proportion of new primary production sustained by aerosol-N deposition was 22.4%, a factor of ten higher than the study average. These data suggest that enhanced N-deposition from polluted continental air masses could sustain pulses of surface ocean biological productivity during periods of dissolved N depletion.

Freshening leads to a three-decade trend of declining nutrients in the western Arctic Ocean

Rapid warming and sea-ice loss in the Arctic Ocean are among the most profound climatic changes to have occurred in recent decades on Earth. Arctic Ocean biological production appears that it may be increasing as a result, but the consequences for nutrient concentrations are unknown. We have assembled a collection of historical field data showing that average concentrations of the macronutrients nitrate and phosphate have decreased by 79% and 29%, respectively, in surface waters of the western Arctic Ocean basin over the past three decades. The field observations and results from numerical ocean simulations suggest that this long-term trend toward more oligotrophic (nutrient-poor) conditions is driven primarily by the compound effects of sea-ice loss: a reduced resupply of nutrients from subsurface waters (due to fresh water addition and stronger upper-ocean stratification) coincident with increased biological consumption of nutrients (due to the greater availability of light needed for photosynthesis).

Sequential changes in ocean circulation and biological export productivity during the last glacial–interglacial cycle: a model–data study

Abstract. We conduct a model–data analysis of the marine carbon cycle to understand and quantify the drivers of atmospheric CO2 concentration during the last glacial–interglacial cycle. We use a carbon cycle box model, “SCP-M”, combined with multiple proxy data for the atmosphere and ocean, to test for variations in ocean circulation and Southern Ocean biological export productivity across marine isotope stages spanning 130 000 years ago to the present. The model is constrained by proxy data associated with a range of environmental conditions including sea surface temperature, salinity, ocean volume, sea-ice cover and shallow-water carbonate production. Model parameters for global ocean circulation, Atlantic meridional overturning circulation and Southern Ocean biological export productivity are optimized in each marine isotope stage against proxy data for atmospheric CO2, δ13C and Δ14C and deep-ocean δ13C, Δ14C and CO32-. Our model–data results suggest that global overturning circulation weakened during Marine Isotope Stage 5d, coincident with a ∼ 25 ppm fall in atmospheric CO2 from the last interglacial period. There was a transient slowdown in Atlantic meridional overturning circulation during Marine Isotope Stage 5b, followed by a more pronounced slowdown and enhanced Southern Ocean biological export productivity during Marine Isotope Stage 4 (∼ −30 ppm). In this model, the Last Glacial Maximum was characterized by relatively weak global ocean and Atlantic meridional overturning circulation and increased Southern Ocean biological export productivity (∼ −20 ppm during MIS 3 and MIS 2). Ocean circulation and Southern Ocean biological export productivity returned to modern values by the Holocene period. The terrestrial biosphere decreased by 385 Pg C in the lead-up to the Last Glacial Maximum, followed by a period of intense regrowth during the last glacial termination and the Holocene (∼ 600 Pg C). Slowing ocean circulation, a colder ocean and to a lesser extent shallow carbonate dissolution contributed ∼ −70 ppm to atmospheric CO2 in the ∼ 100 000-year lead-up to the Last Glacial Maximum, with a further ∼ −15 ppm contributed during the glacial maximum. Our model results also suggest that an increase in Southern Ocean biological export productivity was one of the ingredients required to achieve the Last Glacial Maximum atmospheric CO2 level. We find that the incorporation of glacial–interglacial proxy data into a simple quantitative ocean transport model provides useful insights into the timing of past changes in ocean processes, enhancing our understanding of the carbon cycle during the last glacial–interglacial period.

The Importance of the Phytoplankton “Middle Class” to Ocean Net Community Production

AbstractThe net balance between photosynthesis and respiration in the surface ocean is a key regulator of ocean‐atmosphere carbon dioxide (CO2) partitioning, and by extension, Earth's climate. The slight excess of photosynthesis over community respiration in sunlit waters, known as net community production (NCP), sets the upper bound on the sequestration of carbon via biologically mediated export. Prevailing paradigms suggest a high/low binary where net primary production (NPP), NCP, and export are highest in ecosystems characterized by microplankton (>20 μm) and lowest in ecosystems dominated by picoplankton (<2 μm). This bifurcation model neglects the potential importance of nanoplankton (2–20 μm)—i.e., the “middle” size class—toward global biological pump functioning. Here, we show a relationship between the biomass of nanoplankton and oxygen‐based estimates of NCP across natural ecological gradients in the North Pacific Ocean. Using a suite of high‐resolution optical imaging approaches including SeaFlow, Imaging FlowCytobot, and laser‐based scattering, nanoplankton dynamics are observed to dominate the particle size distribution throughout an ~1,000 km transition between the subtropical and subpolar North Pacific, where NCP rates are threefold to fivefold higher than subtropical values. Based on ecological theory applied to the Darwin size‐based ecosystem model, we hypothesize that intermediate size class organisms are capable of high rates of production via an optimization of bottom‐up and top‐down control inherent to the “middle class.” More broadly, the model indicates the global importance of nanoplankton for ocean biological production.

Assessment of the zonal asymmetry trend in Antarctic total ozonecolumn using TOMS measurements and CCMVal-2 models

In the paper the seasonal trends in the zonal asymmetry in the quasi-stationary wave pattern of total ozone column (TOC) at southern polar latitudes have been investigated. We evaluated and compared seasonal trends in the zonal TOC asymmetry from modern era satellite measurements using the Total Ozone Mapping Spectrometer data and the second Chemistry Climate Model Validation (CCMVal-2) assessment. The model longitude phase shifts in asymmetry are in general consistent with the eastward phase shifts observed in historical period 1979–2005, however, there are underestimated values in individual seasons. Future trends in zonal asymmetry from the eleven CCMVal-2 models up to 2100 are presented. They demonstrate the appearance of reverse (westward) future phase shifts, mainly in austral summer. The results are in agreement with previous study and highlight that the general eastward/westward phase shift is caused by both greenhouse gases changes and ozone depletion/recovery. The greenhouse gases change drives a basic long-term eastward shift, which is enhanced (decelerates or reverses) in austral spring and summer by ozone depletion (recovery). The trends in the TOC asymmetry are forced by a general strengthening of the stratospheric zonal flow, which is interacting with the asymmetry of the Antarctic continent to displace the quasi-stationary wave-1 pattern and thus influences the TOC distribution. The results will be useful in prediction of seasonal anomalies in ozone hole and long-term changes in the local TOC trends, in ultraviolet radiation influence on the Southern Ocean biological productivity and in regional surface climate affected by the zonally asymmetric ozone hole.

Monitoring bio-optical response of coastal waters surrounding the Indian subcontinent to atmospheric dust deposition using satellite data.

The paper investigates the impact of atmospheric dust deposition on ocean biological productivity in association with oceanic supply of nutrients over specific regions of the Arabian Sea (20°N, 69°E) and the Bay of Bengal (20°N, 87°E) during wintertime (November-March) from the year 2012 to 2017 using satellite-based observations. During winter, selected regions are characterized by higher Chlorophyll-a (Chl-a) and major oceanic vertical supply of nutrients. Moderate Resolution Imaging Spectroradiometer onboard Aqua space-platform is used to obtain Chl-a and aerosol optical depth (AOD) data. Blended Chl-a daily product from various satellite sensors is also used. There are a total of nine cases (seven cases of the Arabian Sea region and two cases of the Bay of Bengal region) where episodic Chl-a enhancements following high AOD values are observed. Chl-a maxima lag behind AOD maxima by 1 to 4days. Modern-Era Retrospective analysis for Research and Applications (Version-2) is used for AOD and dust deposition flux estimation. Estimated dust deposition flux ranges between 0.44 and 27.68mgm-2day-1.

Coupled climate–carbon cycle simulation of the Last Glacial Maximum atmospheric CO<sub>2</sub> decrease using a large ensemble of modern plausible parameter sets

Abstract. During the Last Glacial Maximum (LGM), atmospheric CO2 was around 90 ppmv lower than during the pre-industrial period. The reasons for this decrease are most often elucidated through factorial experiments testing the impact of individual mechanisms. Due to uncertainty in our understanding of the real system, however, the different models used to conduct the experiments inevitably take on different parameter values and different structures. In this paper, the objective is therefore to take an uncertainty-based approach to investigating the LGM CO2 drop by simulating it with a large ensemble of parameter sets, designed to allow for a wide range of large-scale feedback response strengths. Our aim is not to definitely explain the causes of the CO2 drop but rather explore the range of possible responses. We find that the LGM CO2 decrease tends to predominantly be associated with decreasing sea surface temperatures (SSTs), increasing sea ice area, a weakening of the Atlantic Meridional Overturning Circulation (AMOC), a strengthening of the Antarctic Bottom Water (AABW) cell in the Atlantic Ocean, a decreasing ocean biological productivity, an increasing CaCO3 weathering flux and an increasing deep-sea CaCO3 burial flux. The majority of our simulations also predict an increase in terrestrial carbon, coupled with a decrease in ocean and increase in lithospheric carbon. We attribute the increase in terrestrial carbon to a slower soil respiration rate, as well as the preservation rather than destruction of carbon by the LGM ice sheets. An initial comparison of these dominant changes with observations and paleoproxies other than carbon isotope and oxygen data (not evaluated directly in this study) suggests broad agreement. However, we advise more detailed comparisons in the future, and also note that, conceptually at least, our results can only be reconciled with carbon isotope and oxygen data if additional processes not included in our model are brought into play.

Has Nitrogen Supply to Coral Reefs in the South Pacific Ocean Changed Over the Past 50 Thousand Years?

AbstractTropical islands can facilitate surface ocean biological productivity by enhancing the supply of nitrogen to the euphotic zone. Yet in the world's most oligotrophic oceanic region, the South Pacific Subtropical Gyre (SPSG), this “island mass effect” appears diminished. If this is the case, where did island coral reefs in the SPSG get their nitrogen from, and has this changed over time? Here we use coral skeleton isotopes (δ15N and δ18O) and element/Ca ratios to identify the sources of nitrogen to a late Pleistocene coral reef in the SPSG (Cook Islands); we then compare these results to modern corals in the same region. The supply of nitrogen to the late Pleistocene reef appears dominated by upwelling of subsurface nitrogen during cool dry events, supplemented with nitrogen from island‐induced N2 fixation (27 ± 3%) during warm wet periods. For the modern corals, N2 fixation supplies nitrogen to the island reefs during cool dry periods with groundwater providing nitrogen during wet periods. We propose that the subsurface supply of nitrogen to the modern reefs has declined as a result of reduced upwelling but this supply has been replaced with increasing nitrogen discharge from groundwater and an increase in island‐induced N2 fixation.

Submesoscale hotspots of productivity and respiration: Insights from high-resolution oxygen and fluorescence sections

Modeling studies have shown that mesoscale and submesoscale processes can stimulate phytoplankton productivity and export production. Here, we present observations from an undulating, towed Video Plankton Recorder (VPR-II) in the tropical Atlantic. The VPR-II collected profiles of oxygen, fluorescence, temperature and salinity in the upper 140m of the water column at a spatial resolution of 1m in the vertical and <2km in the horizontal. The data reveal remarkable "hotspots", i.e. locations 5–10km wide which have elevated fluorescence and decreased oxygen, both of which are likely the result of intense submesoscale upwelling. Based on estimates of source water, estimated from identical temperature and salinity surfaces, hotspots are more often areas of net respiration than areas of net production — although the inferred changes in oxygen are subject to uncertainty in the determination of the source of the upwelled waters since the true source water may not have been sampled. We discuss the spatial distribution of these hotspots and present a conceptual model outlining their possible generation and decline. Simultaneous measurements of O2/Ar in the mixed layer from a shipboard mass spectrometer provide estimates of rates of surface net community production. We find that the subsurface biological hotspots are often expressed as an increase in mixed layer rates of net community production. Overall, the large number of these hotspots support the growing evidence that submesoscale processes are important drivers in upper ocean biological production.

Redox-controlled carbon and phosphorus burial: A mechanism for enhanced organic carbon sequestration during the PETM

Geological records reveal a major perturbation in carbon cycling during the Paleocene–Eocene Thermal Maximum (PETM, ∼56 Ma), marked by global warming of more than 5 °C and a prominent negative carbon isotope excursion of at least 2.5‰ within the marine realm. The entire event lasted about 200,000 yr and was associated with a massive release of light carbon into the ocean–atmosphere system over several thousands of years. Here we focus on the terminal stage of the PETM, during which the ocean–atmosphere system rapidly recovered from the carbon cycle perturbation. We employ a carbon-cycle box model to examine the feedbacks between surface ocean biological production, carbon, oxygen, phosphorus, and carbonate chemistry during massive CO2 release events, such as the PETM. The model results indicate that the redox-controlled carbon–phosphorus feedback is capable of producing enhanced organic carbon sequestration during large carbon emission events. The locale of carbon oxidation (ocean vs. atmosphere) does not affect the amount of carbon sequestered. However, even though the model produces trends consistent with oxygen, excess accumulation rates of organic carbon (∼1700 Pg C during the recovery stage), export production and δ13C data, it fails to reproduce the magnitude of change of sediment carbonate content and the CCD over-deepening during the recovery stage. The CCD and sediment carbonate content overshoot during the recovery stage is muted by a predicted increase in CaCO3 rain. Nonetheless, there are indications that the CaCO3 export remained relatively constant during the PETM. If this was indeed true, then an initial pulse of 3,000 Pg C followed by an additional, slow leak of 2,500 Pg C could have triggered an accelerated nutrient supply to the surface ocean instigating enhanced organic carbon export, consequently increasing organic carbon sequestration, resulting in an accelerated restoration of ocean–atmosphere biogeochemistry during the termination phase of the PETM.

Subregional characterization of mesoscale eddies across the Brazil‐Malvinas Confluence

AbstractHorizontal and vertical motions associated with coherent mesoscale structures, including eddies and meanders, are responsible for significant global transports of many properties, including heat and mass. Mesoscale vertical fluxes also influence upper ocean biological productivity by mediating the supply of nutrients into the euphotic layer, with potential impacts on the global carbon cycle. The Brazil‐Malvinas Confluence (BMC) is a western boundary current region in the South Atlantic with intense mesoscale activity. This region has an active role in the genesis and transformation of water masses and thus is a critical component of the Atlantic meridional overturning circulation. The collision between the Malvinas and Brazil Currents over the Patagonian shelf/slope creates an energetic front that translates offshore to form a vigorous eddy field. Recent improvements in gridded altimetric sea level anomaly fields allow us to track BMC mesoscale eddies with high spatial and temporal resolutions using an automated eddy tracker. We characterize the eddies across fourteen 5° × 5° subregions. Eddy‐centric composites of tracers and geostrophic currents diagnosed from a global reanalysis of surface and in situ data reveal substantial subregional heterogeneity. The in situ data are also used to compute the evolving quasi‐geostrophic vertical velocity (QG‐ω) associated with each instantaneous eddy instance. The QG‐ω eddy composites have the expected dipole patterns of alternating upwelling/downwelling, however, the magnitude and sign of azimuthally averaged vertical velocity varies among subregions. Maximum eddy values are found near fronts and sharp topographic gradients. In comparison with regional eddy composites, subregional composites provide refined information about mesoscale eddy heterogeneity.

Paleoclimatic Signals from the Proxy Records of the Southern Ocean: A Review

Antarctic sea-ice extent and oceanic frontal systems are of primary importance to the marine biota since their marked meridional gradients in physical and chemical properties strongly regulate the phytoplankton contribution to primary productivity.Antarctic sea ice extent along with the Southern Ocean (SO) biological productivity varied considerably during glacial-interglacial periods, and both are known to have played a considerable role in regulating atmospheric CO2 variations in the past. In the present review paper, we seek to understand the past latitudinal variability of the Southern Ocean frontal systems and Antarctic sea-ice extent based on a multi-proxy approach. The first aspect of this paper concentrates on the diatom based reconstructions of paleo sea-ice and hydrographic changes in the Southern Ocean and its impact on diatom sizes and productivity. Secondly, emphasis would be placed on the studies based on the morphology and isotopic composition of foraminifera and its paleoceanographic implication. The foraminifera shell preserved in the sediments provide unparalleled archives of morphological change, faunal variations, and habitat characteristics as a result of hydrographic changes. To sum up, the advantage of a mulitproxy approach including the magnetic, geochemical and sedimentological parameters have been discussed in understanding the Southern Ocean paleoclimate.

Glacial‐interglacial variability in diatom abundance and valve size: Implications for Southern Ocean paleoceanography

AbstractAntarctic sea ice extent along with Southern Ocean biological productivity varied considerably during glacial‐interglacial periods, and both are known to have played a considerable role in regulating atmospheric CO2 variations in the past. Here we present data on diatom absolute abundance (valves/g of sediment) and size over the past ~ 42 ka B.P. and how they link to glacial‐interglacial changes in Antarctic sea ice extent, Southern Ocean frontal systems, and aeolian dust flux. Our records of sea ice and permanent open ocean zone diatom abundances suggest a shift in the Antarctic winter sea ice limit and Polar Front respectively up to the modern‐day Polar Frontal Zone during marine isotopic stages (MIS) 2 and late MIS 3. In addition to glacial shifts in the Polar Front, diatom assemblages also recorded a plausible northward shifts in Polar Front during few intervals of MIS 1. Glacial periods north of the Polar Front in the Indian sector of the Southern Ocean were characterized by higher total diatom abundance, larger Fragilariopsis kerguelensis apical length, and Thalassiosira lentiginosa radius. This is probably a consequence of (1) a northward expansion of the opal belt, a region characterized by high production and export of biogenic silica; (2) an increase in terrigenous input, via erosion of Crozet Islands; and (3) the alleviation of iron deficit by high input of Fe‐bearing dust. The larger and highly silicified diatoms such as F. kerguelensis and T. lentiginosa may have mainly contributed in transporting biogenic silica and organic carbon to the seabed for the last 42 ka, in the northern Polar Frontal Zone of the Indian sector of the Southern Ocean.

Evaluating Southern Ocean biological production in two ocean biogeochemical models on daily to seasonal timescales using satellite chlorophyll and O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; / Ar observations

Abstract. We assess the ability of ocean biogeochemical models to represent seasonal structures in biomass and net community production (NCP) in the Southern Ocean. Two models are compared to observations on daily to seasonal timescales in four different sections of the region. We use daily satellite fields of chlorophyll (Chl) as a proxy for biomass and in situ observations of O2 and Ar supersaturation (ΔO2 / Ar) to estimate NCP. ΔO2 / Ar is converted to the flux of biologically generated O2 from sea to air (O2 bioflux). All data are aggregated to a climatological year with a daily resolution. To account for potential regional differences within the Southern Ocean, we conduct separate analyses of sections south of South Africa, around the Drake Passage, south of Australia, and south of New Zealand. We find that the models simulate the upper range of Chl concentrations well, underestimate spring levels significantly, and show differences in skill between early and late parts of the growing season. While there is a great deal of scatter in the bioflux observations in general, the four sectors each have distinct patterns that the models pick up. Neither model exhibits a significant distinction between the Australian and New Zealand sectors and between the Drake Passage and African sectors. South of 60° S, the models fail to predict the observed extent of biological O2 undersaturation. We suggest that this shortcoming may be due either to problems with the ecosystem dynamics or problems with the vertical transport of oxygen.

A Low-Cost Fluorescent Sensor for pCO2 Measurements

Global warming is believed to be caused by increasing amounts of greenhouse gases (mostly CO2) discharged into the environment by human activity. In addition to an increase in environmental temperature, an increased CO2 level has also led to ocean acidification. Ocean acidification and rising temperatures have disrupted the water’s ecological balance, killing off some plant and animal species, while encouraging the overgrowth of others. To minimize the effect of global warming on local ecosystem, there is a strong need to implement ocean observing systems to monitor the effects of anthropogenic CO2 and the impacts thereof on ocean biological productivity. Here, we describe the development of a low-cost fluorescent sensor for pCO2 measurements. The detector was exclusively assembled with low-cost optics and electronics, so that it would be affordable enough to be deployed in great numbers. The system has several novel features, such as an ideal 90° separation between excitation and emission, a beam combiner, a reference photodetector, etc. Initial tests showed that the system was stable and could achieve a high resolution despite the low cost.

Spatiotemporal variability of MODIS-Aqua-derived aerosol and its impact on surface chlorophyll-a in the Indian coastal and offshore waters

Observation was made to understand the spectral behavior of aerosol from the NASA moderate resolution imaging spectroradiometer (MODIS-Aqua) sensor data. The 869 nm central wavelength band MODIS-Aqua-derived images were retrieved and interpreted. The aerosol concentration was found to be higher during December (Northeast monsoon) over the Bay of Bengal (BoB). Its characterized distinct plume with higher aerosol optical depth (AOD) ranged between 0.05 and 0.25. The coastal aerosol concentration was higher than the offshore region, with decreasing trend (AOD∼0.05 ). To understand aerosol’s link with ocean biological productivity in terms of phytoplankton pigment, the ocean surface chlorophyll was retrieved from 8-years monthly datasets (2002 to 2010). MODIS-Aqua-derived monthly scale aerosol and chlorophyll data were analyzed in the northern Indian Ocean (NIO) region during July 2002 to June 2010. Their interrelationship was found to be significant (r 2 =0.663 and 0.584) over pockets in the BoB and Arabian Sea. Land to seaward movement of wind and aerosol plays a significant role to increasing ocean productivity. This study is very important and interesting to understand the precise composition of aerosol dust like iron (Fe) and increase in sulfate ions, nitrates, etc., and their link to phytoplankton biomass and the related productivity in the Indian water and attempted for the first time in the NIO water.

Atmospheric dissolved iron deposition to the global oceans: effects of oxalate-promoted Fe dissolution, photochemical redox cycling, and dust mineralogy

Abstract. Mineral dust deposition is suggested to be a significant atmospheric supply pathway of bioavailable iron (Fe) to Fe-depleted surface oceans. In this study, mineral dust and dissolved Fe (Fed) deposition rates are predicted for March 2009 to February 2010 using the 3-D chemical transport model GEOS-Chem implemented with a comprehensive dust-Fe dissolution scheme. The model simulates Fed production during the atmospheric transport of mineral dust, taking into account inorganic and organic (oxalate)-promoted Fe dissolution processes, photochemical redox cycling between ferric (Fe(III)) and ferrous (Fe(II)) forms of Fe, dissolution of three different Fe-containing minerals (hematite, goethite, and aluminosilicates), and detailed mineralogy of wind-blown dust from the major desert regions. Our calculations suggest that during the year-long simulation ~0.26 Tg (1 Tg = 1012 g) of Fed was deposited to global oceanic regions. Compared to simulations only taking into account proton-promoted Fe dissolution, the addition of oxalate and Fe(II)/Fe(III) redox cycling to the dust-Fe mobilization scheme increased total annual model-predicted Fed deposition to global oceanic regions by ~75%. The implementation of Fe(II)/Fe(III) photochemical redox cycling in the model also allows for the distinction between different oxidation states of deposited Fed. Our calculations suggest that during the daytime, large fractions of Fed deposited to the global oceans is likely to be in Fe(II) form, while nocturnal fluxes of Fed are largely in Fe(III) form. Model sensitivity simulations suggest Fed fluxes to the oceans can range from ~50% reduction to ~150% increase associated with the uncertainty in Fe-containing minerals commonly found in dust particles. This study indicates that Fed deposition to the oceans is controlled by total dust-Fe mass concentrations, mineralogy, the surface area of dust particles, atmospheric chemical composition, cloud processing, and meteorological parameters and exhibits complex and spatiotemporally variable patterns. Our study suggests that the explicit model representation of individual processes leading to Fed production within mineral dust are needed to improve the understanding of the atmospheric Fe cycle, and quantify the effect of dust-Fe on ocean biological productivity, carbon cycle, and climate.

Factors challenging our ability to detect long-term trends in ocean chlorophyll

Abstract. Global climate change is expected to affect the ocean's biological productivity. The most comprehensive information available about the global distribution of contemporary ocean primary productivity is derived from satellite data. Large spatial patchiness and interannual to multidecadal variability in chlorophyll a concentration challenges efforts to distinguish a global, secular trend given satellite records which are limited in duration and continuity. The longest ocean color satellite record comes from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), which failed in December 2010. The Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color sensors are beyond their originally planned operational lifetime. Successful retrieval of a quality signal from the current Visible Infrared Imager Radiometer Suite (VIIRS) instrument, or successful launch of the Ocean and Land Colour Instrument (OLCI) expected in 2014 will hopefully extend the ocean color time series and increase the potential for detecting trends in ocean productivity in the future. Alternatively, a potential discontinuity in the time series of ocean chlorophyll a, introduced by a change of instrument without overlap and opportunity for cross-calibration, would make trend detection even more challenging. In this paper, we demonstrate that there are a few regions with statistically significant trends over the ten years of SeaWiFS data, but at a global scale the trend is not large enough to be distinguished from noise. We quantify the degree to which red noise (autocorrelation) especially challenges trend detection in these observational time series. We further demonstrate how discontinuities in the time series at various points would affect our ability to detect trends in ocean chlorophyll a. We highlight the importance of maintaining continuous, climate-quality satellite data records for climate-change detection and attribution studies.

MESMO 2: a mechanistic marine silica cycle and coupling to a simple terrestrial scheme

Abstract. Here we describe the second version of Minnesota Earth System Model for Ocean biogeochemistry (MESMO 2), an earth system model of intermediate complexity, which consists of a dynamical ocean, dynamic-thermodynamic sea ice, and energy moisture balanced atmosphere. The new version has more realistic land ice masks and is driven by seasonal winds. A major aim in version 2 is representing the marine silica cycle mechanistically in order to investigate climate-carbon feedbacks involving diatoms, a critically important class of phytoplankton in terms of carbon export production. This is achieved in part by including iron, on which phytoplankton uptake of silicic acid depends. Also, MESMO 2 is coupled to an existing terrestrial model, which allows for the exchange of carbon, water and energy between land and the atmosphere. The coupled model, called MESMO 2E, is appropriate for more complete earth system simulations. The new version was calibrated, with the goal of preserving reasonable interior ocean ventilation and various biological production rates in the ocean and land, while simulating key features of the marine silica cycle.