scholarly journals Offshore Wind Farm Footprint on Organic and Mineral Particle Flux to the Bottom

2021 ◽  
Vol 8 ◽  
Author(s):  
Evgeny Ivanov ◽  
Arthur Capet ◽  
Emil De Borger ◽  
Steven Degraer ◽  
Eric J. M. Delhez ◽  
...  
Keyword(s):  
Organic Carbon ◽  
Water Column ◽  
Particle Deposition ◽  
Blue Mussel ◽  
Transport Model ◽  
Offshore Wind ◽  
Deposition Flux ◽  
Mineral Particle ◽  
Gravel Beds ◽  
The Impact

Offshore wind farms (OWFs) are an important source of renewable energy accounting for 2.3% of the European Union's electricity demand. Yet their impact on the environment needs to be assessed. Here, we couple a hydrodynamic (including tides and waves) and sediment transport model with a description of the organic carbon and mineral particle dynamics in the water column and sediments. The model is applied to the Belgian Coastal Zone (BCZ) where OWFs currently occupy 7% of its surface area which is estimated to double in the next 5 years. The impact of OWFs on the environment is represented through the filtration of the water column and fecal pellets production by the blue mussel, the dominant fouling organism. Our model simulations show that the impact of biodeposition on the mud particle sedimentation and on sediment composition is small compared to the fluxes associated with tidal deposition and resuspension and the lateral inputs. In contrast, the total organic carbon (TOC) flux to the sediment is significantly altered inside the OWF perimeters and TOC deposition is increased up to 50% in an area 5 km around the monopiles. Further away, the TOC flux to the bottom decreases with a notable effect up to 30 km away. The major changes are found along the direction of the main residual current and tidal ellipse's major axis. In addition, sub-mesoscale gyres act as retention areas with increased carbon deposition. A future OWF in the BCZ will be located close to gravel beds in a Natura 2000 area, considered as vulnerable habitats and biodiversity hotspots. The different scenarios for this OWF, varying in turbine number and positioning, are compared in terms of impact on the carbon and mineral particle deposition flux in the BCZ and, particularly, to these gravel beds. The scenarios show that the number of turbines has only a slight impact on the TOC deposition flux, unlike their positioning that significantly alters the TOC flux to the gravel beds. The TOC deposition flux exceeds 50%, when the turbines are placed next to the gravel beds; while a limited increase is simulated, when the turbines are located the farthest possible from them.

Modelling of the offshore wind farm footprint on organic and mineral particle deposition flux

2021 ◽  
Author(s):  
Evgeny Ivanov ◽  
Arthur Capet ◽  
Emil De Borger ◽  
Steven Degraer ◽  
Eric Delhez ◽  
...  
Keyword(s):  
Organic Carbon ◽  
Particle Deposition ◽  
Blue Mussel ◽  
Transport Model ◽  
Offshore Wind ◽  
Deposition Flux ◽  
Mineral Particle ◽  
Southern Bight ◽  
Gravel Beds ◽  
The Impact

<p>Being an important source of renewable energy, offshore wind farms (OWFs) are currently flourishing in European coastal seas, with a largely unknown long-term impact on the environment. By providing hard substrate habitat to fouling species (such as the blue mussel), who filter water and excrete rapidly sinking fecal pellets, OWFs change the sediment composition and its carbon balance through biodeposition. </p><p>Here we coupled a hydrodynamic model (including tides), a wave model and a sediment transport model with a description of organic carbon dynamics. The coupled model was run for the Southern Bight of the North Sea under different scenarios: i) no OWFs; ii)  current OWF placement; and iii) several scenarios for future OWF placement in a new concession area, that differ in the number of installed monopiles and their placements.</p><p>Simulations showed that the tidal remobilization of mineral particles by the dominant current is orders of magnitude higher than their biodeposition from the OWFs. The total organic carbon (TOC) flux, however, appeared to be highly altered (up to 50%) by OWF biodeposition, especially in 5 km vicinity of the monopiles. At a greater distance (5 - 30 km away from the monopiles), the TOC biodeposition flux decreases. The majors alteration in the TOC flux is aligned with the major axis of the regional tidal current and the main direction of the residual current, with local residual gyres acting as TOC traps.</p><p>A future OWF, whose current concession zone overlaps a protected Natura 2000 area with its gravel beds acting as biodiversity hotspots, is expected to affect them through TOC biodeposition flux alteration. However, the magnitude of the impact appeared to be strongly dependent on the monopile placement, and very little on the number of monopiles. The gravel beds will experience a 50% TOC influx increase, if the monopiles are placed over them or just next to them, but already at 3 km distance this increase would be less than 10 %.</p>

The Carbon:²³⁴Thorium ratios of sinking particles in the California Current Ecosystem 2: Examination of a thorium sorption, desorption, and particle transport model

2019 ◽  
Author(s):  
Michael Stukel ◽  
Thomas Kelly
Keyword(s):  
Organic Carbon ◽  
Water Column ◽  
Particulate Organic Carbon ◽  
Transport Model ◽  
California Current ◽  
In Situ Measurements ◽  
Power Law Distribution ◽  
Sinking Particles ◽  
Organic Carbon Concentration

Thorium-234 (234Th) is a powerful tracer of particle dynamics and the biological pump in the surface ocean; however, variability in carbon:thorium ratios of sinking particles adds substantial uncertainty to estimates of organic carbon export. We coupled a mechanistic thorium sorption and desorption model to a one-dimensional particle sinking model that uses realistic particle settling velocity spectra. The model generates estimates of 238U-234Th disequilibrium, particulate organic carbon concentration, and the C:234Th ratio of sinking particles, which are then compared to in situ measurements from quasi-Lagrangian studies conducted on six cruises in the California Current Ecosystem. Broad patterns observed in in situ measurements, including decreasing C:234Th ratios with depth and a strong correlation between sinking C:234Th and the ratio of vertically-integrated particulate organic carbon (POC) to vertically-integrated total water column 234Th, were accurately recovered by models assuming either a power law distribution of sinking speeds or a double log normal distribution of sinking speeds. Simulations suggested that the observed decrease in C:234Th with depth may be driven by preferential remineralization of carbon by particle-attached microbes. However, an alternate model structure featuring complete consumption and/or disaggregation of particles by mesozooplankton (e.g. no preferential remineralization of carbon) was also able to simulate decreasing C:234Th with depth (although the decrease was weaker), driven by 234Th adsorption onto slowly sinking particles. Model results also suggest that during bloom decays C:234Th ratios of sinking particles should be higher than expected (based on contemporaneous water column POC), because high settling velocities minimize carbon remineralization during sinking.

Biogeochemical and ecological features of sinking particulate matter in the deep Ionian Sea (E. Mediterranean) during a 10-year time series study: impacts of atmospheric and oceanographic variabilities on carbon production and sequestration

2021 ◽  
Author(s):  
Alexandra Gogou ◽  
Constantine Parinos ◽  
Spyros Stavrakakis ◽  
Emmanouil Proestakis ◽  
Maria Kanakidou ◽  
...  
Keyword(s):  
Particulate Matter ◽  
Organic Carbon ◽  
Carbon Cycle ◽  
Atmospheric Deposition ◽  
Water Column ◽  
Particulate Organic Carbon ◽  
Nutrient Dynamics ◽  
Ionian Sea ◽  
Ecological Features ◽  
The Impact

<p>Biotic and abiotic processes that form, alter, transport, and remineralize particulate organic carbon, silicon, calcium carbonate, and other minor and trace chemical species in the water column are central to the ocean’s ecological and biogeochemical functioning and of fundamental importance to the ocean carbon cycle. Sinking particulate matter is the major vehicle for exporting carbon from the sea surface to the deep sea. During its transit towards the sea floor, most particulate organic carbon (POC) is returned to inorganic form and redistributed in the water column. This redistribution determines the surface concentration of dissolved CO<sub>2</sub>, and hence the rate at which the ocean can absorb CO<sub>2</sub> from the atmosphere. The ability to predict quantitatively the depth profile of remineralization is therefore critical to deciphering the response of the global carbon cycle to natural and human-induced changes.</p><p>Aiming to investigate the significant biogeochemical and ecological features and provide new insights on the sources and cycles of sinking particulate matter, a mooring line of five sediment traps was deployed from 2006 to 2015 (with some gap periods) at 5 successive water column depths (700, 1200, 2000, 3200 and 4300 m) in the SE Ionian Sea, northeastern Mediterranean (‘NESTOR’ site). We have examined the long-term records of downward fluxes for Corg, N<sub>tot</sub>, δ<sup>13</sup>Corg and δ<sup>15</sup>N<sub>tot</sub>, along with the associated ballast minerals (opal, lithogenics and CaCO<sub>3</sub>), lipid biomarkers, Chl-a and PP rates, phytoplankton composition, nutrient dynamics and atmospheric deposition.  </p><p>The satellite-derived seasonal and interannual variability of phytoplankton metrics (biomass and phenology) and atmospheric deposition (meteorology and air masses origin) was examined for the period of the sediment trap experiment. Regarding the atmospheric deposition, synergistic opportunities using Earth Observation satellite lidar and radiometer systems are proposed (e.g. Cloud‐Aerosol Lidar with Orthogonal Polarization - CALIOP, Moderate Resolution Imaging Spectroradiometer - MODIS), aiming towards a four‐dimensional exploitation of atmospheric aerosol loading (e.g. Dust Optical Depth) in the study area.</p><p>Our main goals are to: i) develop a comprehensive knowledge of carbon fluxes and associated mineral ballast fluxes from the epipelagic to the mesopelagic and bathypelagic layers, ii) elucidate the mechanisms governing marine productivity and carbon export and sequestration to depth and iii) shed light on the impact of atmospheric forcing and deposition in respect to regional and large scale circulation patterns and climate variability and the prevailing oceanographic processes (internal variability).</p><p>Acknowledgments</p><p>We acknowledge support of this work by the Action ‘National Network on Climate Change and its Impacts – <strong>CLIMPACT</strong>’, funded by the Public Investment Program of Greece (GSRT, Ministry of Development and Investments).</p>

scholarly journals Organic matter mineralization and trace element post-depositional redistribution in Western Siberia thermokarst lake sediments

2011 ◽  
Vol 8 (11) ◽  
pp. 3341-3358 ◽  
Author(s):  
S. Audry ◽  
O. S. Pokrovsky ◽  
L. S. Shirokova ◽  
S. N. Kirpotin ◽  
B. Dupré
Keyword(s):  
Trace Elements ◽  
Organic Matter ◽  
Organic Carbon ◽  
Lake Sediments ◽  
Water Column ◽  
Western Siberia ◽  
Ecosystem Development ◽  
Thermokarst Lake ◽  
Fe And Mn ◽  
The Impact

Abstract. This study reports the very first results on high-resolution sampling of sediments and their porewaters from three thermokarst (thaw) lakes representing different stages of ecosystem development located within the Nadym-Pur interfluve of the Western Siberia plain. Up to present time, the lake sediments of this and other permafrost-affected regions remain unexplored regarding their biogeochemical behavior. The aim of this study was to (i) document the early diagenesic processes in order to assess their impact on the organic carbon stored in the underlying permafrost, and (ii) characterize the post-depositional redistribution of trace elements and their impact on the water column. The estimated organic carbon (OC) stock in thermokarst lake sediments of 14 ± 2 kg m−2 is low compared to that reported for peat soils from the same region and denotes intense organic matter (OM) mineralization. Mineralization of OM in the thermokarst lake sediments proceeds under anoxic conditions in all the three lakes. In the course of the lake development, a shift in mineralization pathways from nitrate and sulfate to Fe- and Mn-oxyhydroxides as the main terminal electron acceptors in the early diagenetic reactions was suggested. This shift was likely promoted by the diagenetic consumption of nitrate and sulfate and their gradual depletion in the water column due to progressively decreasing frozen peat lixiviation occurring at the lake's borders. Trace elements were mobilized from host phases (OM and Fe- and Mn-oxyhydroxides) and partly sequestered in the sediment in the form of authigenic Fe-sulfides. Arsenic and Sb cycling was also closely linked to that of OM and Fe- and Mn-oxyhydroxides. Shallow diagenetic enrichment of particulate Sb was observed in the less mature stages. As a result of authigenic sulfide precipitation, the sediments of the early stage of ecosystem development were a sink for water column Cu, Zn, Cd, Pb and Sb. In contrast, at all stages of ecosystem development, the sediments were a source of dissolved Co, Ni and As to the water column. However, the concentrations of these trace elements remained low in the bottom waters, indicating that sorption processes on Fe-bounding particles and/or large-size organo-mineral colloids could mitigate the impact of post-depositional redistribution of toxic elements on the water column.

scholarly journals Changes in dissolved iron deposition to the oceans driven by human activity: a 3-D global modelling study

2015 ◽  
Vol 12 (5) ◽  
pp. 3943-3990
Author(s):  
S. Myriokefalitakis ◽  
N. Daskalakis ◽  
N. Mihalopoulos ◽  
A. R. Baker ◽  
A. Nenes ◽  
...  
Keyword(s):  
Air Quality ◽  
Transport Model ◽  
Organic Ligand ◽  
Photochemical Reactions ◽  
Anthropogenic Emissions ◽  
Spatial And Temporal Variability ◽  
Deposition Flux ◽  
Chemical Transport Model ◽  
The Future ◽  
The Impact

Abstract. The global atmospheric iron (Fe) cycle is parameterized in the global 3-D chemical transport model TM4-ECPL to simulate the proton- and the organic ligand-promoted mineral Fe dissolution as well as the aqueous-phase photochemical reactions between the oxidative states of Fe(III/II). Primary emissions of total (TFe) and dissolved (DFe) Fe associated with dust and combustion processes are also taken into account. TFe emissions are calculated to amount to ~35 Tg Fe yr−1. The model reasonably simulates the available Fe observations, supporting the reliability of the results of this study. Accounting for proton- and organic ligand-promoted Fe-dissolution in present-day TM4-ECPL simulations, the total Fe-dissolution is calculated to be ~0.163 Tg Fe yr−1 that accounts for up to ~50% of the calculated total DFe emissions. The atmospheric burden of DFe is calculated to be ~0.012 Tg Fe. DFe deposition presents strong spatial and temporal variability with an annual deposition flux ~0.489 Tg Fe yr−1 from which about 25% (~0.124 Tg Fe yr−1) are deposited over the ocean. The impact of air-quality on Fe deposition is studied by performing sensitivity simulations using preindustrial (year 1850), present (year 2008) and future (year 2100) emission scenarios. These simulations indicate that an increase (~2 times) in Fe-dissolution may have occurred in the past 150 years due to increasing anthropogenic emissions and thus atmospheric acidity. On the opposite, a decrease (~2 times) of Fe-dissolution is projected for near future, since atmospheric acidity is expected to be lower than present-day due to air-quality regulations of anthropogenic emissions. The organic ligand contribution to Fe dissolution shows inverse relationship to the atmospheric acidity thus its importance has decreased since the preindustrial period but is projected to increase in the future. The calculated changes also show that the atmospheric DFe supply to High-Nutrient-Low-Chlorophyll oceanic areas (HNLC) characterized by Fe scarcity, has increased (~50%) since the preindustrial period. However, the DFe deposition flux is expected to decrease (~30%) to almost preindustrial levels over the Northern Hemisphere HNLC oceanic regions in the future. Significant reductions of ~20% over the Southern Ocean and the remote tropical Pacific Ocean are also projected which can further limit the primary productivity over HNLC waters.

The relationship between sediment and water quality, and riverine sediment loads in the wave-dominated estuaries of south-west Western Australia

2004 ◽  
Vol 55 (6) ◽  
pp. 581 ◽  
Author(s):  
L. C. Radke ◽  
I. P. Prosser ◽  
M. Robb ◽  
B. Brooke ◽  
D. Fredericks ◽  
...  
Keyword(s):  
Water Quality ◽  
Organic Matter ◽  
Soil Erosion ◽  
Organic Carbon ◽  
Water Column ◽  
Nitrogen And Phosphorus ◽  
Weathering Index ◽  
Sediment Loads ◽  
The Impact ◽  
Column Total

We examine surface sediment and water column total nutrient and chlorophyll a concentrations for 12 estuaries with average water depths <4 m, and calculated sediment loads ranging from 0.2 to 10.8 kg m−2 year−1. Sediment total nitrogen, phosphorus and organic carbon concentrations vary inversely with sediment loads due to: (i) the influx of more mineral-rich sediment into the estuaries; and (ii) increasing sediment sulfidation. Sediment total organic carbon (TOC) : total sulfur (TS) and TS : Fe(II) ratios correlated to sediment loads because enhanced sedimentation increases burial, hence the importance of sulfate reduction in organic matter degradation. Curvilinear relationships were found between a weathering index and organic matter δ13C in sediment, and sediment load. The rising phase of the curve (increasing weathering, lighter isotopic values) at low to intermediate loads relates to soil erosion, whereas regolith or bedrock erosion probably explains the declining phase of the curve (decreasing weathering, heavier isotopic values) at higher sediment loads. The pattern of change for water column total nutrients (nitrogen and phosphorus) with sediment loads is similar to that of the weathering index. Most water quality problems occur in association with soil erosion, and at sediment loads that are intermediate for the estuaries studied. Limited evidence is presented that flushing can moderate the impact of sediment loads upon the estuaries.

scholarly journals Phosphorus Dynamics Associated With Organic Carbon Mineralization by Reduction of Sulfate and Iron in Sediment Exposed to Fish Farming

2021 ◽  
Vol 8 ◽  
Author(s):  
Jin-Sook Mok ◽  
Ayeon Choi ◽  
Bomina Kim ◽  
Sung-Uk An ◽  
Won-Chan Lee ◽  
...  
Keyword(s):  
Organic Carbon ◽  
Water Column ◽  
Fish Farming ◽  
Fish Feed ◽  
Internal Source ◽  
Control Sediment ◽  
P Release ◽  
P Flux ◽  
The Impact ◽  
Total P

The expansion of the aquaculture industry has resulted in accumulation of phosphorus (P)-rich organic matter via uneaten fish feed. To elucidate the impact of fish farming on P dynamics, P speciation, and benthic P release along with partitioning of organic carbon (Corg) mineralization coupled to sulfate reduction (SR) and iron reduction (FeR) were investigated in the sediments from Jinju Bay, off the southern coast of South Korea, in July 2013. SR in the farm sediment was 6.9-fold higher than the control sediment, and depth-integrated (0–10 cm) concentrations of NH4+, PO43–, and H2S in pore water of the farm sediment were 2.2-, 3.3-, and 7.4-fold higher than that in control sediment, respectively. High biogenic-P that comprised 28% of total P directly reflected the impact of P-rich fish feed, which ultimately enhanced the bioavailability (58% of total P) of P in the surface sediment of the farm site. In the farm sediment where SR dominated Corg mineralization, H2S oxidation coupled to the reduction of FeOOH stimulated release of P bound to iron oxide, which resulted in high regeneration efficiency (85%) of P in farm sediments. Enhanced P desorption from FeOOH was responsible for the increase in authigenic-P and benthic P flux. Authigenic-P comprised 33% of total P, and benthic P flux to the overlying water column accounted for approximately 800% of the P required for primary production. Consequently, excessive benthic P release resulting directly from oversupply of P-rich fish feed was a significant internal source of P for the water column, and may induce undesirable eutrophication and harmful algal blooms in shallow coastal ecosystems.

scholarly journals Organic matter mineralization and trace element post-depositional redistribution in Western Siberia thermokarst lake sediments

2011 ◽  
Vol 8 (4) ◽  
pp. 8845-8894
Author(s):  
S. Audry ◽  
O. S. Pokrovsky ◽  
L. S. Shirokova ◽  
S. N. Kirpotin ◽  
B. Dupré
Keyword(s):  
Trace Elements ◽  
Organic Matter ◽  
Organic Carbon ◽  
Lake Sediments ◽  
Water Column ◽  
Western Siberia ◽  
Ecosystem Development ◽  
Thermokarst Lake ◽  
Fe And Mn ◽  
The Impact

Abstract. This study reports the very first results on high-resolution sampling of sediments and their porewaters from three thermokarst (thaw) lakes representing different stages of ecosystem development located within the Nadym-Pur interfluve of the Western Siberia plain. Up to present time, the lake sediments of this and other permafrost-affected regions remain unexplored regarding their biogeochemical behavior. The aim of this study was to (i) document the early diagenesic processes in order to assess their impact on the organic carbon stored in the underlying permafrost, and (ii) characterize the post-depositional redistribution of trace elements and their impact on the water column. The estimated organic carbon (OC) stock in thermokarst lake sediments of 14 ± 2 kg m−2 is low compared to that reported for peat soils from the same region and denotes intense organic matter (OM) mineralization. Mineralization of OM in the thermokarst lake sediments proceeds under anoxic conditions in all the three lakes. In the course of the lake development, a shift in mineralization pathways was evidenced from nitrate and sulfate to Fe- and Mn-oxyhydroxides as the main terminal electron acceptors in the early diagenetic reactions. This shift was promoted by the diagenetic consumption of nitrate and sulfate and their gradual depletion in the water column due to progressively decreasing frozen peat lixiviation occurring at the lake's borders. Trace elements were mobilized from host phases (OM and Fe- and Mn-oxyhydroxides) and partly sequestered in the sediment in the form of authigenic Fe-sulfides. Arsenic and Sb cycling was also closely linked to that of OM and Fe- and Mn-oxyhydroxides. Shallow diagenetic enrichment of particulate Sb was observed in the less mature stages. As a result of authigenic sulfide precipitation, the sediments of the early stage of ecosystem development were a sink for water column Cu, Zn, Cd, Pb and Sb. In contrast, at all stages of ecosystem development, the sediments were a source of dissolved Co, Ni and As to the water column. However, the concentrations of these trace elements remained low in the bottom waters, indicating that sorption processes on Fe-bounding particles and/or large-size organo-mineral colloids could mitigate the impact of post-depositional redistribution of toxic elements on the water column.

scholarly journals Enhancement of marine cloud albedo via controlled sea spray injections: a global model study of the influence of emission rates, microphysics and transport

2010 ◽  
Vol 10 (1) ◽  
pp. 735-761 ◽  
Author(s):  
H. Korhonen ◽  
K. S. Carslaw ◽  
S. Romakkaniemi
Keyword(s):  
Wind Speed ◽  
Particle Deposition ◽  
Transport Model ◽  
Anthropogenic Sources ◽  
Sea Spray ◽  
Emission Rates ◽  
Climatic Effects ◽  
Cloud Drop ◽  
Cloud Albedo ◽  
The Impact

Abstract. Modification of cloud albedo by controlled emission of sea spray particles into the atmosphere has been suggested as a possible geoengineering option to slow global warming. Previous global studies have imposed changes in cloud drop concentration in low level clouds to explore the radiative and climatic effects. Here, we use a global aerosol transport model to quantify how an imposed flux of sea spray particles affects the natural aerosol processes, the particle size distribution, and concentrations of cloud drops. We assume that the proposed fleet of vessels emits sea spray particles with a wind speed-dependent flux into four regions of persistent stratocumulus cloud off the western coasts of continents. The model results show that fractional changes in cloud drop number concentration (CDNC) vary substantially between the four regions because of differences in wind speed (which affects the spray efficiency of the vessels), transport and particle deposition rates, and because of variations in aerosols from natural and anthropogenic sources. Using spray emission rates comparable to those implied by previous studies we find that the predicted CDNC changes are very small (maximum 20%) and in one of the four regions even negative. The weak or negative effect is because the added particles suppress the in-cloud supersaturation and prevent existing aerosol particles from forming cloud drops. A scenario with five times higher emissions (considerably higher than previously assumed) increases CDNC on average by 45–163%, but median concentrations are still below the 375 cm−3 assumed in previous studies. An inadvertent effect of the spray emissions is that sulphur dioxide concentrations are suppressed by 1–2% in the seeded regions and sulphuric acid vapour by 64–68% due to chemical reactions on the additional salt particles. The impact of this suppression on existing aerosol is negligible in the model, but should be investigated further in the real environment so that inadvertent impacts can be excluded.

The Carbon:²³⁴Thorium ratios of sinking particles in the California Current Ecosystem 2: Examination of a thorium sorption, desorption, and particle transport model

2019 ◽  
Author(s):  
Michael Stukel ◽  
Thomas Kelly
Keyword(s):  
Organic Carbon ◽  
Water Column ◽  
Particulate Organic Carbon ◽  
Transport Model ◽  
California Current ◽  
In Situ Measurements ◽  
Power Law Distribution ◽  
Sinking Particles ◽  
Organic Carbon Concentration

Thorium-234 (234Th) is a powerful tracer of particle dynamics and the biological pump in the surface ocean; however, variability in carbon:thorium ratios of sinking particles adds substantial uncertainty to estimates of organic carbon export. We coupled a mechanistic thorium sorption and desorption model to a one-dimensional particle sinking model that uses realistic particle settling velocity spectra. The model generates estimates of 238U-234Th disequilibrium, particulate organic carbon concentration, and the C:234Th ratio of sinking particles, which are then compared to in situ measurements from quasi-Lagrangian studies conducted on six cruises in the California Current Ecosystem. Broad patterns observed in in situ measurements, including decreasing C:234Th ratios with depth and a strong correlation between sinking C:234Th and the ratio of vertically-integrated particulate organic carbon (POC) to vertically-integrated total water column 234Th, were accurately recovered by models assuming either a power law distribution of sinking speeds or a double log normal distribution of sinking speeds. Simulations suggested that the observed decrease in C:234Th with depth may be driven by preferential remineralization of carbon by particle-attached microbes. However, an alternate model structure featuring complete consumption and/or disaggregation of particles by mesozooplankton (e.g. no preferential remineralization of carbon) was also able to simulate decreasing C:234Th with depth (although the decrease was weaker), driven by 234Th adsorption onto slowly sinking particles. Model results also suggest that during bloom decays C:234Th ratios of sinking particles should be higher than expected (based on contemporaneous water column POC), because high settling velocities minimize carbon remineralization during sinking.

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