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Unmixing deep sea sedimentary records identifies sensitivity of marine calcifying zooplankton to abrupt warming and ocean acidification in the past

Posted by mmaheigan

· Tuesday, May 3rd, 2022

Ocean acidification and rising temperatures have led to concerns about how calcifying organisms foundational to marine ecosystems, will be affected in the near future. We often look to analogous abrupt climate change events in Earth’s geologic past to inform our predictions of these future communities. The Paleocene-Eocene thermal maximum (PETM) is an apt analog for modern climate change. The PETM was a global warming and ocean acidification event driven by geologically abrupt changes to the global carbon cycle approximately 56 million years ago. Much of what we know about the PETM is from the study of deep sea sedimentary records and the microfossils within them. However, these records can experience intense sediment mixing—from bottom water currents and burrowing by organisms living along the seafloor—which can blur or distort the primary climate and ecological signals in these paleorecords.

PETM corrected foram graphic - see caption for detail

Figure 1. A) Frequency distribution of single-shell stable carbon isotope (δ13C) values for planktic foraminiferal shells from a deep sea sedimentary PETM record from the equatorial Pacific (n = 548). Note that 50% of shells measured record distinctly PETM values, while 49.5% record distinctly pre-PETM values. B) Comparison of diversity metric (Shannon-H) between the isotopically filtered (i.e., unmixed) and unfiltered (i.e., mixed) planktic foraminiferal assemblages.

A recent study in the Proceedings of the National Academy of Sciences used geochemical signatures measured from individual microfossil shells of planktic foraminifera (surface-dwelling marine calcareous zooplankton) to deconvolve the effects of sediment mixing on a deep sea PETM record from the equatorial Pacific. Use of this “isotopic filtering” (unmixing) method revealed that nearly 50% of shells in the PETM interval were reworked contaminants that lived before the global warming event (Figure 1A). The identification and removal of these older shells from fossil census counts drastically changed interpretations of how these organisms responded to the PETM. Prior interpretations assumed that planktic foraminiferal communities living near the equator diversified during the PETM. However, by deconvolving the effects of sediment mixing on the same equatorial deep sea record, researchers found that these communities actually suffered an abrupt decrease in diversity at the onset of the PETM (Figure 1B). This decrease is likely due to several taxa migrating towards the poles to escape the extreme heat of the tropics and lower oxygen conditions found at deeper water depths (i.e., thermocline) during the PETM. Additionally, some taxa may have undergone morphological changes, signaling reduced calcification, in response to extreme acidifying conditions. Today, anthropogenic carbon emission rates are ~10 times faster than the carbon cycling perturbation that triggered the PETM. Although planktic foraminifera and other key zooplankton survived the PETM, these communities suffered at the hands of extreme sea surface temperatures and acidifying waters, and may not be able to cope the rate of environmental changes in our ocean over the coming centuries.

Authors:
Brittany N. Hupp (University of Wisconsin-Madison)
D. Clay Kelly (University of Wisconsin-Madison)
John W. Williams (University of Wisconsin-Madison)

How atmospheric and oceanographic forcing impact the carbon sequestration in an ultra-oligotrophic marine system

Posted by mmaheigan

· Wednesday, August 11th, 2021

Sinking particles are a critical conduit for the export of material from the surface to the deep ocean. Despite their importance in oceanic carbon cycling, little is known about the composition and seasonal variability of sinking particles which reach abyssal depths. Oligotrophic waters cover ~75% of the ocean surface and contribute over 30% of the global marine carbon fixation. Understanding the processes that control carbon export to the deep oligotrophic areas is crucial to better characterize the strength and efficiency of the biological pump as well as to project the response of these systems to climate fluctuations and anthropogenic perturbations.

In a recent study published in Frontiers in Earth Science, authors synthesized data from atmospheric and oceanographic parameters, together with main mass components, and stable isotope and source-specific lipid biomarker composition of sinking particles collected in the deep Eastern Mediterranean Sea (4285m, Ierapetra Basin) for a three-year period (June 2010-June 2013). In addition, this study compared the sinking particulate flux data with previously reported deep-sea surface sediments from the study area to shed light on the benthic–pelagic coupling.

Figure Caption: a) Biplot of net primary productivity vs export efficiency (top and bottom horizontal dashed lines indicate threshold for high and low export efficiency regimes). b) Biplot of POC-normalized concentrations of terrestrial vs. phytoplankton-derived lipid biomarkers of the sinking particles collected in the deep Eastern Mediterranean Sea (Ierapetra Basin, NW Levantine Basin) from June 2010–June 2013, and surface sediments collected from January 2007 to June 2012 in the study area.

Both seasonal and episodic pulses are crucial for POC export to the deep Eastern Mediterranean Sea. POC fluxes peaked in spring April–May 2012 (12.2 mg m⁻² d⁻¹) related with extreme atmospheric forcing. Overall, summer particle export fuels more efficient carbon sequestration than the other seasons. The results of this study highlight that the combination of extreme weather events and aerosol deposition can trigger an influx of both marine labile carbon and anthropogenic compounds to the deep. Finally, the comparison of the sinking particles flux data with surface sediments revealed an isotopic discrimination, as well as a preferential degradation of labile organic matter during deposition and burial, along with higher preservation of land-derived POC in the underlying sediments. This study provides key knowledge to better understand the export, fate and preservation vs. degradation of organic carbon, and for modeling the organic carbon burial rates in the Mediterranean Sea.

Authors:
Rut Pedrosa-Pamies (The Ecosystems Center, Marine Biological Laboratory, US; Research Group in Marine Geosciences, University of Barcelona, Spain)
Constantine Parinos (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)
Anna Sanchez-Vidal (Group in Marine Geosciences, University of Barcelona, Spain)
Antoni Calafat (Group in Marine Geosciences, University of Barcelona, Spain)
Miquel Canals (Group in Marine Geosciences, University of Barcelona, Spain)
Dimitris Velaoras (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)
Nikolaos Mihalopoulos (Environmental Chemical Processes Laboratory, University of Crete; National Observatory of Athens, Greece)
Maria Kanakidou (Environmental Chemical Processes Laboratory, University of Crete Greece)
Nikolaos Lampadariou (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)
Alexandra Gogou (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)

Sea ice loss amplifies CO2 increase in the Arctic

Posted by mmaheigan

· Thursday, January 7th, 2021

Warming and sea ice loss over the past few decades have caused major changes in sea surface partial pressure of CO₂ (pCO₂) of the western Arctic Ocean, but detailed temporal variations and trends during this period of rapid climate-driven changes are not well known.

Based on an analysis of an international Arctic pCO₂ synthesis data set collected between 1994-2017, the authors of a recent paper published in Nature Climate Change observed that summer sea surface pCO₂ in the Canada Basin is increasing at twice the rate of atmospheric CO₂ rise. Warming, ice loss and subsequent CO₂ uptake in the Basin are amplifying seasonal pCO₂ changes, resulting in a rapid long-term increase. Consequently, the summer air-sea CO₂ gradient has decreased sharply and may approach zero by the 2030s, which is reducing the basin’s capacity to remove CO₂from the atmosphere. In stark contrast, sea surface pCO₂ on the Chukchi Shelf remains low and relatively constant during this time frame, which the authors attribute to increasingly strong biological production in response to higher intrusion of nutrient-rich Pacific Ocean water onto the shelf as a result of increased Bering Strait throughflow. These trends suggest that, unlike the Canada Basin, the Chukchi Shelf will become a larger carbon sink in the future, with implications for the deep ocean carbon cycle and ecosystem.

As Arctic sea ice melting accelerates, more fresh, low-buffer capacity, high-CO₂ water will enter the upper layer of the Canada Basin, which may rapidly acidify the surface water, endanger marine calcifying organisms, and disrupt ecosystem function.

Figure. 1: TOP) Sea surface pCO₂ trend in the Canada Basin and Chukchi Shelf. The grey dots represent the raw observations of pCO₂, black dots are the monthly mean of pCO₂ at in situ SST, and red dots are the monthly means of pCO₂ normalized to the long-term means of SST. The arrows indicate the statistically significant change in ∆pCO₂. BOTTOM) Sea ice-loss amplifying surface water pCO₂ in the Canada Basin. Black dots represent the initial condition for pCO₂ and DIC at -1.6 ℃. The arrows indicate the processes of warming (red), CO₂ uptake from the atmosphere (green), dilution by ice meltwater (blue). The yellow shaded areas indicate the possible seasonal variations of pCO₂, which are amplified by the synergistic effect of ice melt, warming and CO₂ uptake.

Authors:
Zhangxian Ouyang (University of Delaware, USA),
Di Qi (Third Institute of Oceanography, China),
Liqi Chen (Third Institute of Oceanography, China),
Taro Takahashi† (Columbia University, USA),
Wenli Zhong (Ocean University of China, China),
Michael D. DeGrandpre (University of Montana, USA),
Baoshan Chen (University of Delaware, USA),
Zhongyong Gao (Third Institute of Oceanography, China),
Shigeto Nishino (Japan Agency for Marine-Earth Science and Technology, Japan),
Akihiko Murata (Japan Agency for Marine-Earth Science and Technology, Japan),
Heng Sun (Third Institute of Oceanography, China),
Lisa L. Robbins (University of South Florida, USA),
Meibing Jin (International Arctic Research Center, USA),
Wei-Jun Cai* (University of Delaware, USA)

Physics vs. biology in Southern Ocean nutrient gradients

Posted by mmaheigan

· Tuesday, June 16th, 2020

In the Southern Ocean, surface water silicate (SiO₄) concentrations decline very quickly relative to nitrate concentrations along a northward gradient toward mode water formation regions on the northern edge (Figure 1a, b). These mode waters play a critical role in driving global nutrient concentrations, setting the biogeochemistry of low- and mid-latitude regions around the globe after they upwell further north. To explain this latitudinal surface gradient, most hypotheses have implicated diatoms, which take up and export silicon as well as nitrogen: (1) Diatoms, including highly-silicified species such as Fragilariopsis kerguelensis, are more abundant in the Southern Ocean than elsewhere; (2) Iron limitation, which is prevalent in the Southern Ocean, elevates the Si:N ratio of diatoms; (3) Mass export of empty diatom frustules pumps silicate but not nitrate to deeper waters.

Figure 1: (a) and (b) nitrate and silicate concentrations in surface waters of the Southern Ocean (GLODAPv2_2019 data). (c) Model results of a standard run (black diamonds), a run without biology (red diamonds) and a run without mixing (blue diamonds).

In a recent paper published in Biogeosciences, the authors use an idealized model to explore the relative roles of biological vs. physical processes in driving the observed latitudinal surface nutrient gradients. Over timescales of a few years, removing the effects of biology (no SiO₄uptake or export) from the model elevates silicate concentrations slightly over the entire latitudinal range, but does not remove the strong latitudinal gradient (Figure 1c). However, if the effects of vertical mixing processes such as upwelling and entrainment are removed from the model by eliminating the observed deep [SiO₄] gradient, the observed surface nutrient gradient is greatly altered (Figure 1c). These model results suggest that, over short timescales, physics is more important than biology in driving the observed surface water gradient in SiO₄:NO₃ ratios and forcing silicate depletion of mode waters leaving the Southern Ocean. These findings add to our understanding of Southern Ocean dynamics and the downstream effects on other oceans.

Authors:
P. Demuynck (University of Southampton)
T. Tyrrell (University of Southampton)
A.C. Naveira Garabato (University of Southampton)
C.M. Moore (University of Southampton)
A.P. Martin (National Oceanography Centre)

A Methane-Charged Carbon Pump in Shallow Marine Sediments

Posted by mmaheigan

· Wednesday, June 3rd, 2020

Ocean margins are often characterized by the transport of methane, a potent greenhouse gas, entering from the subsurface and moving towards the seafloor. However, a significant portion of subsurface methane is consumed within shallow sediments via microbial driven anaerobic oxidation of methane (AOM). AOM converts the methane carbon to dissolved inorganic carbon (DIC) and reduces the amount of sulfate that diffuses down from the seafloor towards a sediment interval known as the sulfate-methane transition zone (SMTZ). The SMTZ is where the upward flux of methane encounters the downward diffusive sulfate flux (Figure 1). While the mechanisms of methane production and consumption have been extensively studied, the fate of the DIC that is produced in methane-charged sediments is not well constrained.

In a recent study published in Frontiers in Marine Science, authors used existing reports of methane and sulfate flux values to the SMTZ and synthesized a carbon flow model to quantify the DIC cycling in diffusive methane flux sites globally. They report an annual average of 8.7 Tmol (1 Tmol = 10¹² moles) of DIC entering the diffusive methane-charged shallow marine sediments due to sulfate reduction coupled with AOM and organic matter degradation, as well as DIC input from depth (Figure 1). Approximately 75% (average of 6.5 Tmol year^–1) of this DIC pool flows upward toward the water column, making it a potential contributor to oceanic CO₂and ocean acidification. Further, an average of 1.7 Tmol year^–1DIC precipitates as methane-derived authigenic carbonates. This synthesis emphasizes the importance of the SMTZ, not only as a methane sink but also an important biogeochemical front for global DIC cycling.

Figure 1: A simplified representation of DIC cycling at diffusive methane charged settings.

The study highlights that regions characterized by diffusive methane fluxes can contribute significantly to the oceanic inorganic carbon pool and sedimentary carbonate accumulation. DIC outflux from the methane-charged sediments is comparable to ~20% global riverine DIC flux to oceans. Methane-derived authigenic carbonate precipitation is comparable to ~15% of carbonate accumulation on continental shelves and in pelagic sediments, respectively. These pathways must be included in coastal and geologic carbon models.

Authors:
Sajjad Akam (Texas A&M University-Corpus Christi)
Richard Coffin (Texas A&M University-Corpus Christi)
Hussain Abdulla (Texas A&M University-Corpus Christi)
Timothy Lyons (University of California, Riverside)

Little big exporters

Posted by mmaheigan

· Wednesday, April 8th, 2020

In the Southern Ocean, coccolithophores are thought to account for a major fraction of marine carbonate production and export to the deep sea. Despite their importance in the ocean carbon cycle, we lack fundamental information about Southern Ocean coccolithophore abundance, species composition, and contribution to carbonate export.

Figure caption: Heliscosphaera carteri (left), Coccolithus pelagicus (right) and Emiliania huxleyi (bottom right, partially behind C. pelagicus) coccospheres retrieved from the subantarctic waters south of Tasmania. Image Ruth Eriksen, courtesy AAD EMU.

A recent study in Biogeosciences has generated annual observations of coccolithophore species composition and contribution to calcium carbonate fluxes at two sites that are representative of a large portion of the Subantarctic zone. Coccolithophores account for roughly half of the annual calcium carbonate exported to the deep sea. Notably, it is not the most abundant species (Emiliania huxleyi), but rather the less abundant and larger species (e.g. Calcidiscus leptoporus, Helicosphaera carteri and Coccolithus pelagicus) that make the greatest contribution to carbonate export to the deep sea. Since these larger species exhibit substantially different ecological traits from the opportunistic E. huxleyi, predictions of future response of Southern Ocean coccolithophore communities should not be based on the physiological results from experiments with E. huxleyi. Rather, new physiological response experiments of those less abundant, larger coccolithophore species are urgently needed to constrain responses of these important carbonate exporters to environmental change in the Southern Ocean. This study underscores the importance of phytoplankton ecological traits on the regulation of the marine carbon cycle and emphasizes the need for more species-specific studies to improve predictions of marine ecosystem response to ongoing climate change.

Authors
Andrés S. Rigual Hernández (Universidad de Salamanca)
Thomas W. Trull (CSIRO and ACE CRC)
Scott D. Nodder (NIWA)
José A. Flores (Universidad de Salamanca)
Helen Bostock (University of Queensland,)
Fátima Abrantes (Portuguese Institute for Sea and Atmosphere and CCMAR)
Ruth S. Eriksen (CSIRO and IMAS)
Francisco J. Sierro (Universidad de Salamanca)
Diana M. Davies (CSIRO and ACE CRC)
Anne-Marie Ballegeer (Universidad de Salamanca)
Miguel A. Fuertes (Universidad de Salamanca)
Lisa C. Northcote (NIWA)

Tiny, but effective: Gelatinous zooplankton and the ocean biological carbon pump

Posted by mmaheigan

· Wednesday, March 25th, 2020

Barely visible to the naked eye, gelatinous zooplankton play important roles in marine food webs. Cnidaria, Ctenophora, and Urochordata are omnipresent and provide important food sources for many more highly developed marine organisms. These small, nearly transparent organisms also transport large quantities of “jelly-carbon” from the upper ocean to depth. A recent study in Global Biogeochemical Cycles focused on quantifying the gelatinous zooplankton contribution to the ocean carbon cycle.

Figure 1. Processes and pathways or gelatinous carbon transfer to the deep ocean.

Using >90,000 data points (1934 to 2011) from the Jellyfish Database Initiative (JeDI), the authors compiled global estimates of jellyfish biomass, production, vertical migration, and jelly carbon transfer efficiency. Despite their small biomass relative to the total mass of organisms living in the upper ocean, their rapid, highly efficient sinking makes them a globally significant source of organic carbon for deep-ocean ecosystems, with 43-48% of their upper ocean production reaching 2000 m, which translates into 0.016 Pg C yr^-1.

Figure 2. Mass deposition event of jellyfish at 3500 m in the Arabian Sea (Billett et al. 2006).

Sediment trap data have suggested that carbon transport associated with large, episodic gelatinous blooms in localized open ocean and continental shelf regions could often exceed phytodetrital sources, in particular instances. These mass deposition events and their contributions to deep carbon export must be taken into account in models to better characterize marine ecosystems and reduce uncertainties in our understanding of the ocean’s role in the global carbon cycle.

Links:

Jellyfish Database Initiative http://jedi.nceas.ucsb.edu, http://jedi.nceas.ucsb.edu-dmo.org/dataset/526852 )

Authors:
Mario Lebrato (Christian‐Albrechts‐University Kiel and Bazaruto Center for Scientific Studies, Mozambique)
Markus Pahlow (GEOMAR)
Jessica R. Frost (South Florida Water Management District)
Marie Küter (Christian‐Albrechts‐University Kiel)
Pedro de Jesus Mendes (Marine and Environmental Scientific and Technological Solutions, Germany)
Juan‐Carlos Molinero (GEOMAR)
Andreas Oschlies (GEOMAR)

Hurricane-driven surge of labile carbon into the deep North Atlantic Ocean

Posted by mmaheigan

· Thursday, February 27th, 2020

Tropical cyclones (hurricanes and typhoons) are the most extreme episodic weather event affecting subtropical and temperate oceans. Hurricanes generate intense surface cooling and vertical mixing in the upper ocean, resulting in nutrient upwelling into the photic zone and episodic phytoplankton blooms. However, their influence on the deep ocean is unknown.

Figure 1. (a) Particulate organic carbon (POC) flux and percentage of the total mass flux (yellow) (top panel); fluxes (middle panel) and POC-normalized concentrations (bottom panel) of diagnostic lipid biomarkers for phytoplankton-derived and labile material, zooplankton, bacteria, and other (see legend); (b) Lipid concentrations (left panel) and POC-normalized concentrations (right panel) of diagnostic lipid biomarkers for the same sources as in (a) (see legend) measured two weeks after Nicole’s passage (25-29 Oct. 2016). Shown for reference are total lipid concentration profiles in April 2015 (dark gray, typical post spring bloom conditions) and Nov 2015 (light gray, typical minimum production period).

In October 2016, Category 3 Hurricane Nicole passed over the Bermuda time-series site (Oceanic Flux Program (OFP) and Bermuda Atlantic Time-Series site (BATS)) in the oligotrophic NW Atlantic Ocean. In a recent study published in Geophysical Research Letters, authors synthesized multidisciplinary data from hydrographic and phytoplankton measurements and lipid composition of sinking and suspended particles collected from OFP and BATS, respectively, after Hurricane Nicole in 2016. After the hurricane passed, particulate fluxes of lipids diagnostic of fresh phytodetritus, zooplankton, and microbial biomass increased by 30-300% at 1500 m depth and 30-800% at 3200 m depth (Figure 1a). In addition, mesopelagic suspended particles were enriched in phytodetrital material, as well as zooplankton- and bacteria-sourced lipids (Figure 1b), indicating particle disaggregation and a deep-water ecosystem response.

These results suggest that carbon export and biogeochemical cycles may be impacted by climate-induced changes in hurricane frequency, intensity, and tracks, and, underscore the sensitivity of deep ocean ecosystems to climate perturbations.

Authors:
Rut Pedrosa-Pamies (Marine Biological Laboratory)
Maureen H. Conte (Bermuda Institute of Ocean Science and Marine Biological Laboratory)
JC Weber (Marine Biological Laboratory)
Rodney Johnson (Bermuda Institute of Ocean Science)

Unexpected DOC additions in the deep Atlantic

Posted by mmaheigan

· Tuesday, January 7th, 2020

Oceanic dissolved organic carbon (DOC) ultimately exchanges with atmospheric CO₂ and thus represents an important carbon source/sink with consequence for climate. Most of the DOC is recalcitrant to microbial degradation, with some fractions surviving for thousands of years. Therefore, DOC in the deep ocean was thought to be stable or to decrease slowly over decades to centuries due to biotic and abiotic sinks. However, a study published in Global Biogeochemical Cycles shows that there are some zones of the deep Atlantic Ocean where recalcitrant DOC experiences net production. Using data from oceanographic cruises across the Atlantic Ocean, the authors first identified the major water masses in the basin and the percentage of each in every sample taken for DOC analysis. The study revealed net additions of 27 million tons of dissolved organic carbon per year in the deep South Atlantic. On the other hand, the North Atlantic serves as a net sink, removing 298 million tons of carbon annually. DOC production observed in the deep Atlantic is probably due to the sinking particles that solubilize into DOC, since DOC enrichment was most evident at latitudes characterized as elevated productivity divergence zones.

Figure 1. Water masses along GO-SHIP line A16 (colored dots) and recalcitrant DOC variations due to biogeochemical processes (black dots within each water mass) in the deep Atlantic Ocean. Water mass domains are defined as the set of samples with the corresponding water mass proportion ≥50%. Recalcitrant DOC latitudinal variations per water stratum due to biogeochemical processes (ΔDOC) is in μmol kg^-1. Numbers on the plots are DOC values for the corresponding dots. Scales (not shown) are the same for all the plots, from -4 to 6 μmol kg^-1. Positive (negative) ΔDOC indicates values higher (lower) than the average DOC calculated for each water mass using an optimum multiparameter (OMP) analysis. DOC = dissolved organic carbon. AAIW = Antarctic Intermediate Water; UNADW = upper North Atlantic Deep Water; ISOW = Iceland Scotland Overflow Water; CDW = Circumpolar Deep Water; WSDW = Weddell Sea Deep Water. Figure created with Ocean Data View (Schlitzer, 2015).

Considering that the net DOC production over the entire Atlantic basin euphotic zone is 0.70–0.75 Pg C year^-1, the authors estimated that 30–39% of that DOC is consumed in the deep Atlantic subsequent to its export by overturning circulation. The upper North Atlantic Deep Water (UNADW) acts as the primary sink, accounting for 66% of the recalcitrant DOC removal in the North Atlantic. Conversely, the Antarctic Intermediate Water (AAIW) is the primary recipient, with 45% of recalcitrant DOC production in the South Atlantic, closely followed by the old UNADW that gains 44% of the recalcitrant DOC in the southern basin.

The Atlantic works as a mosaic of water masses, where both removal and addition of recalcitrant DOC occurs, with the dominant term dependent on the origin, temperature, age and depth of the water masses. The production of recalcitrant DOC in the deep ocean should be considered in biogeochemical models dealing with the carbon cycle and climate.

Authors:
C. Romera-Castillo and J. L. Pelegrí (Instituto de Ciencias del Mar, CSIC, Spain)
M. Álvarez (Instituto Español de Oceanografía, Spain)
D. A. Hansell (University of Miami, USA)
X. A. Álvarez-Salgado (Instituto de Investigaciones Marinas, CSIC, Spain)

What really controls deep-seafloor calcite dissolution?

Posted by mmaheigan

· Monday, December 16th, 2019

On time scales of tens to millions of years, seawater acidity is primarily controlled by biogenic calcite (CaCO₃) dissolution on the seafloor. Our quantitative understanding of future oceanic pH and carbonate system chemistry requires knowledge of what controls this dissolution. Past experiments on the dissolution rate of suspended calcite grains have consistently suggested a high-order, nonlinear dependence on undersaturation that is independent of fluid flow rate. This form of kinetics has been extensively adopted in models of deep-sea calcite dissolution and pH of benthic sediments. However, stirred-chamber and rotating-disc dissolution experiments have consistently demonstrated linear kinetics of dissolution and a strong dependence on fluid flow velocity. This experimental discrepancy surrounding the kinetic control of seafloor calcite dissolution precludes robust predictions of oceanic response to anthropogenic acidification.

In a recent study published in Geochimica et Cosmochimica Acta, authors have reconciled these divergent experimental results through an equation for the mass balance of the carbonate ion at the sediment-water interface (SWI), which equates the rate of production of that ion via dissolution and its diffusion in sediment porewaters to the transport across the diffusive sublayer (DBL) at the SWI. If the rate constant derived from suspended-grain experiments is inserted into this balance equation, the rate of carbonate ion supply to the SWI from the sediment (sediment-side control) is much greater in the oceans than the rate of transfer across the DBL (water-side control). Thus, calcite dissolution at the seafloor, while technically under mixed control, is strongly water-side dominated. Consequently, a model that neglects boundary-layer transport (sediment-side control alone) invariably predicts CaCO₃-versus-depth profiles that are too shallow compared to available data (Figure 1). These new findings will inform future attempts to model the ocean’s response to acidification.

Figure 1: Plots of the calcite (CaCO₃) content of deep-sea sediments as a function of oceanic depth. Left panel: data from the Northwestern Atlantic Ocean. Right panel: data from the Southwest Pacific Ocean. The blue line represents predicted CaCO₃ content assuming no boundary-layer effects (pure sediment-side control). The red line is the prediction that includes both sediment and water effects (mixed control), and the green line is the prediction with pure water-side control. The agreement between the red and green lines signifies that calcite dissolution is essentially water-side controlled at the seafloor. These results are duplicated for all tested regions of the oceans.

Authors:
Bernard P. Boudreau (Dalhousie University)
Olivier Sulpis (University of Utrecht)
Alfonso Mucci (McGill University)

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.

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