mmaheigan :: Ocean Carbon & Biogeochemistry

Studying marine ecosystems and biogeochemical cycles in the face of environmental change

Author Archive for mmaheigan – Page 23

Antarctic Ocean CO2 helped end the ice age

Posted by mmaheigan

· Tuesday, April 2nd, 2019

Many scientists have long hypothesized that the ocean around Antarctica was responsible for changing CO₂ levels during ice ages, but lacked definitive evidence. A new study in Nature provides the most direct evidence of this process to date and provides crucial evidence of the mechanisms—including changing sea ice cover and bipolar seesaw (warming in the Southern Hemisphere during cooling in the Northern Hemisphere) events—that controlled CO₂ and climate during the ice ages.

Using samples of fossil deep-sea corals collected from 1000 m in the Drake Passage (Figure 1a), the authors were able to reconstruct the CO₂ content of the deep ocean. They found that the deep ocean CO₂ record was the “mirror image” of CO₂ in the atmosphere (Figure 1b), with the ocean storing CO₂ during an ice age and releasing it back to the atmosphere during deglaciation. CO₂ rise during the last ice age occurred in a series of steps and jumps associated with intervals of rapid climate change.

As well as helping scientists better understand the ice ages, the new findings also provide context to current CO₂ rise and climate change. Although the CO₂ rise that helped end the last ice age was dramatic in geological terms, CO₂ rise due to human activity over the last 100 years is even larger and about 100 times faster. CO₂ rise at the end of the ice age helped drive major melting of ice sheets resulting in sea level rise of >100 meters. These results bolster the idea that if we want to prevent dangerous levels of global warming and sea level rise in the future, we need to reduce CO₂ emissions as quickly as possible

Authors:
J. W. B. Rae (University of St Andrews, UK)
A. Burke (University of St Andrews, UK)
L. F. Robinson (University of Bristol, UK)
J. F. Adkins (California Institute of Technology)
T. Chen (University of Bristol, UK, Nanjing University, China)
C. Cole (University of St Andrews, UK)
R. Greenop (University of St Andrews, UK)
T. Li (University of Bristol, UK, Nanjing University, China)
E. F. M. Littley (University of St Andrews, UK)
D. C. Nita (University of St Andrews, UK, Babes-Bolyai University, Romania)
J. A. Stewart (University of St Andrews, UK, University of Bristol, UK)
B. J. Taylor (University of St Andrews, UK)

A half century perspective: Seasonal productivity and particulates in the Ross Sea

Posted by mmaheigan

· Tuesday, April 2nd, 2019

Studies of cruise observations in the Ross Sea are typically biased to a single or a few year(s), and long-term trends have predominantly come from satellites. Consequently, the in situ climatological patterns of nutrients and particulate matter have remained vague and unclear. What are the typical patterns of nutrients and particulate matter concentrations in the Ross Sea in spring and summer? How do these concentrations affect annual productivity estimates?

Patterns of nutrient and particulate matter in the Ross Sea can play a wide-ranging role in a productive region like the Ross Sea. Smith and Kaufman (2018) recently synthesized austral spring and summer (November to February) observations from 42 Ross Sea research cruises (1967-2016) to analyze broad biogeochemical patterns. The resulting climatologies revealed interesting seasonal patterns of nutrient uptake and particulate organic carbon (POC) to chlorophyll (chl) ratios (POC:chl). Temporal patterns in the nitrate and phosphate climatologies confirm the role of early spring haptophyte (Phaeocystis antarctica) growth, followed by limited nitrogen and phosphorus removal in summer. However, a notable increase in POC occurred later in summer that was largely independent of chlorophyll changes, resulting in a dramatic increase in POC:chl. A gradual decline in silicic acid concentrations throughout the summer, along with an increased occurrence of biogenic silica during this time suggest that diatoms may be responsible for this later POC spike. Revised estimates of primary productivity based on these observed climatological POC:chl ratios suggests that summer blooms may be a significant contributor to seasonal productivity, and that estimates of productivity based on satellite pigments underestimate annual production by at least 70% (Figure 1).

Figure 1. Bio-optical estimates of mean productivity using a constant POC:chl ratio (black dots and lines) and modified estimates of productivity using the monthly climatological POC:chl ratios (red dots and lines), in a) the Ross Sea polynya region and b) the western Ross Sea region.

By clarifying typical seasonal patterns of nutrient uptake and POC:chl, these climatologies underscore the biogeochemical importance of both spring haptophyte growth and previously underestimated summer diatom growth in the Ross Sea. Further investigation of the causes and consequences of elevated summer ratios is needed, as assessments of regional food webs and biogeochemical cycles depend on more accurate understanding of primary productivity patterns. Likewise, these results highlight the need for continued efforts to constrain satellite productivity estimates in the Ross Sea using in situ constituent ratios.

For other relevant work on seasonal biogeochemical patterns in the Ross Sea, please see https://doi.org/10.1016/j.dsr2.2003.07.010. And for intra-seasonal estimates of particulate organic carbon to chlorophyll using gliders, please see: https://doi.org/10.1016/j.dsr.2014.06.011.

Authors:
Walker O. Smith Jr. (VIMS, College of William and Mary)
Daniel E. Kaufman (VIMS, College of William and Mary; now at Chesapeake Research Consortium)

Pteropod populations stable or increasing according to long-term study along the Western Antarctic Peninsula

Posted by mmaheigan

· Thursday, March 21st, 2019

Shelled pteropods (pelagic snails) are abundant planktonic predators and prey, linking grazers and higher trophic levels and contributing to the carbon cycle via consumption and excretion. Pteropods have been heralded as bioindicators of ocean acidification, given their aragonitic shell’s susceptibility to dissolution, which could ultimately lead to declining abundance. However, pteropod population dynamics are understudied, particularly in the Southern Ocean, a region predicted to be highly impacted by both warming and ocean acidification. In a recent publication in Limnology and Oceanography, long-term data sets from the Western Antarctic Peninsula show that while there is considerable interannual variability in pteropod abundance, populations have remained stable over the past 25 years, with some pteropod species (gymnosomes (non-shelled pteropod) overall, L. antarctica and C. pyramidata (shelled pteropods) regionally) even increasing during this period (Figure 1).

Figure 1. Annual pteropod abundance anomalies for the entire Palmer Antarctica Long-Term Ecological Research (LTER) study region along the Western Antarctic Peninsula. (a) Limacina helicina antarctica (shelled pteropod), (b) Gymnosomes – nonshelled pteropods that prey on shelled pteropods (p = 0.007, r² = 0.27), and (c) Clio pyramidata (shelled pteropod). Effect of environment on pteropod abundance. (d) SST vs. L. antarctica abundance, e) Sea ice advance vs. L. antarctica and Gymnosome abundance, (f) Sea ice retreat vs. C. pyramidata abundance. Data plotted are annual anomalies for each year of the time series (1993–2017). Sea ice advance is lagged 2-yr behind pteropod abundance (e.g., 2017 pteropod annual anomaly is plotted against 2015 sea ice advance annual anomaly) SST are lagged 1-yr behind L. antarctica abundance (e.g., 2017 L. antarctica annual anomaly is plotted against 2016 SST). Regression lines for significant linear relationships are shown, regression statistics are as follows: (d) SST vs. L. antarctica (circles): n = 25, p = 0.006, r² = 0.25 (e) sea ice advance vs. L. antarctica (filled-circles) and Gymnosomes (empty-circles): n = 25, p = 0.003, r² = 0.30 (dashed line); (f) sea ice retreat vs. C. pyramidata (squares): n = 14, p = 0.0003, r² = 0.64.

There was no significant influence of carbonate chemistry parameters (e.g., aragonite saturation state) on pteropod abundance, since the Western Antarctic Peninsula has yet to experience prolonged conditions characteristic of ocean acidification. However, other environmental factors such as warming and associated sea ice retreat were more influential. For example, warmer, ice-free waters in one year typically led to higher pteropod abundances the following year, suggesting that pteropods may be better adapted than expected to warming conditions due to climate change. The authors propose that earlier sea ice retreat promotes recruitment and subsequent expansion of pteropods further South, which could explain their increased abundance in this subregion. These results increase our understanding of pteropod responses to environmental variability, which is important for predicting future effects of climate change on regional carbon cycling and plankton trophic interactions in the Southern Ocean.

Authors:
Patricia S. Thibodeau (VIMS)
Deborah K. Steinberg (VIMS)
Sharon E. Stammerjohn (University of Colorado at Boulder)
Claudine Hauri (University of Alaska Fairbanks)

Zooplankton vertical migrations represent a significant source of carbon export in the ocean

Posted by mmaheigan

· Friday, March 15th, 2019

Huge numbers of tiny marine animals, known as zooplankton, migrate between the surface ocean and the twilight zone (200 – 1,000 m below the surface) everyday; it is the largest migration event anywhere on the planet. How much carbon do these animals transport with them and how much do they leave behind sequestered in the deep ocean? In a recent publication in Global Biogeochemical Cycles, Archibald et al. (2019) used a simple model to estimate the magnitude of carbon flux into the twilight zone from zooplankton vertical migrations and observed that it was a significant contributor to carbon export on a global scale. The study also revealed strong regional patterns in migration-mediated carbon flux, with the greatest impact on export occurring at subtropical latitudes (Figure 1).

Figure 1. Percent increase in the modeled carbon export flux out of the surface ocean as a result of zooplankton vertical migrations.

Migrating zooplankton also consume significant amounts of oxygen at depth, generating a local maximum in the oxygen utilization profile at depth within the migration layer. By including the effect of the metabolism of migrating zooplankton, the model is able to produce a more detailed picture of oxygen utilization over the twilight zone. The model in this study effectively simulates the complex phenomenon of zooplankton vertical migrations, providing a simple framework that will allow researchers to investigate how this key component of the global carbon cycle might change under future climatic conditions. For example, if increased stratification leads to a greater representation of small cells in phytoplankton communities, then zooplankton migration-mediated carbon export is expected to make up a proportionally larger fraction of the total carbon export flux.

Authors:
Kevin M. Archibald (Woods Hole Oceanographic Institution and Massachusetts Institute of Technology)
David A. Siegel (University of California, Santa Barbara)
Scott C. Doney (University of Virginia)

How fast are elements sinking in the ocean?

Posted by mmaheigan

· Tuesday, March 5th, 2019

The sinking of elements in the ocean influences many important processes such as deep ocean carbon storage and the availability of trace metals for phytoplankton. Previously, quantification of this sinking flux has been done using sediment trap deployments or tracer measurements of a particle-reactive radioisotope. Since sediment traps and each particular radioisotope each have caveats in how they quantify sinking flux, sinking particulate flux measurements, especially trace metal fluxes, are especially sparse, with relatively large uncertainties. For the first time ever, in the U.S. GEOTRACES North Atlantic campaign (GA03), four types of radioisotope data (thorium-234, polonium-210, thorium-228 and thorium-230) were measured, along with a periodic table’s worth of particulate elements that can be used to quantify sinking fluxes at locations with prior sediment trap studies, including the Ocean Flux Program (OFP), for comparison.

Sinking flux estimates of particulate organic carbon (POC) and particulate iron (pFe) derived using different methods, including the different radionuclides labelled and sediment traps from oceanic sites near Bermuda. These include the Bermuda-Atlantic Time-series site (BATS), the Ocean Flux Program site (OFP), and the Bermuda Rise (BaRFlux site). The GA03 and BaRFlux data represent observations from 2012 and 2013. The triangles and stars represent data from throughout the time-series observations of those sites.

In a new study published in Global Biogeochemical Cycles, a team of collaborators synthesized all of the radioisotope and particle composition measurements from the GA03 cruise, as well as results from a nearby study called BaRFlux, to constrain sinking fluxes of carbon and eight trace elements (P, Cd, Co, Cu, Mn, Al, Fe and thorium-232) throughout the North Atlantic Ocean. The five different methods for constraining flux (sediment traps plus the four radioisotope methods) agree encouragingly well given the independent uncertainties associated with each method. Additionally, since the four radioisotopes have a range in half-lives from days to thousands of years, the different methods can reconstruct particle fluxes throughout the water column, from the dynamic bloom-and-bust-like changes near the surface to the relatively slow, long-term sinking into the abyssal ocean. These fluxes will improve the understanding of the global budgets of carbon and trace elements. This study would not have been possible without the support of OCB and GEOTRACES who co-funded a synthesis workshop on biogeochemical cycling of trace elements at the Lamont-Doherty Earth Observatory in summer 2016.

Also see Eos highlight on this article

Authors:
Christopher T. Hayes (University of Southern Mississippi)
Erin E. Black (Woods Hole Oceanographic Institution, now at Dalhousie University)
Robert F. Anderson (Lamont-Doherty Earth Observatory of Columbia University)
Mark Baskaran (Wayne State University)
Ken O. Buesseler (Woods Hole Oceanographic Institution)
Matthew A. Charette (Woods Hole Oceanographic Institution)
Hai Cheng (Xi’an Jiaotong University and University of Minnesota)
Kirk Cochran (Stony Brook University)
Lawrence Edwards (University of Minnesota)
Patrick Fitzgerald (Stony Brook University)
Phoebe J. Lam (University of California Santa Cruz
Yanbin Lu (Earth Observatory of Singapore)
Stephanie O. Morris (Woods Hole Oceanographic institution)
Daniel C. Ohnemus (Bigelow Laboratory for Ocean Sciences, now at Skidaway Institute of Oceanography)
Frank J. Pavia (Lamont-Doherty Earth Observatory of Columbia University)
Gillian Stewart (Queens College, City University of New York)
Yi Tang (Queens College, City University of New York)

You better repeat it: Serial ocean acidification experiments on fish early life stages

Posted by mmaheigan

· Tuesday, March 5th, 2019

To detect potential effects of acidification on marine organisms, experimenters most commonly use within-experiment replication, but repeating the experiments themselves is rarely done. While the first approach suffices to detect major CO₂ effects, other potentially important responses may get detected and robustly quantified only via serial experimentation. A study by Baumann et al. in Biology Letters comprises a meta-analysis of 20 standard CO₂ exposure experiments conducted over six years on Atlantic silverside (Menidia menidia) offspring.

Figure 1: Robust estimate of silverside CO₂ sensitivity based on serial experimentation. (A, B) Mean CO₂ effect size calculated as the log-transformed response ratio of six early life history traits measured at 20 standard experiments between 2012-2017 (Error: bootstrapped 95% confidence intervals). (C) Seasonal change in CO₂ sensitivity in silverside early life stages. Each symbol represents an individual experiment, using offspring obtained by fertilizing wild spawners throughout their spring/summer spawning season.

Silversides are an abundant and ecologically important forage fish in the North Atlantic. The study revealed that during early life stages, Atlantic silversides tolerate pCO₂levels up to ~2,000 µatm, with seasonal shifts in sensitivity. However, this early exposure to high pCO₂ levels reduces embryo survival by 9% and overall survival by 13% (Figure 1). Future ocean acidification could cause reduced survival of these and other forage fish, and thus impact their diverse marine predators, including seabirds and commercially important fish species. This sustained experimental work resulted in the most robustly constrained estimates of average CO₂ effect sizes for a marine organism to date, demonstrating the utility of serial experimentation as a powerful tool for assessing organism responses to changing CO₂.

Authors:
Hannes Baumann
Emma L. Cross
Chris S. Murray
(all University of Connecticut)

Gulf of Mexico: A blue carbon hotspot of mangroves, seagrass and marshes

Posted by mmaheigan

· Wednesday, February 20th, 2019

The Gulf of Mexico (GoM) is an important global hotspot that comprises over 2.1615 million hectares of blue carbon habitats, including mangroves, seagrasses, and salt marshes, which collectively store 480.5 Tg of organic carbon (C_org) just in the upper 1 meter of sediment. Some of these important areas of carbon sequestration are protected or conserved, but much of the area is vulnerable, as 69 million people (US and Mexico) live within 50 miles of these blue carbon habitats, so the potential for development and subsequent habitat loss is high. In a recent study published in Science of the Total Environment, the estuaries around the GoM were delineated to determine areal extent and associated carbon stocks for all three habitats.

Figure 1: Map of blue carbon extent and stock for six sub-regions in the Gulf of Mexico estuaries and the Florida Shelf. The areal extent in hectares (ha) and associated organic carbon (C_org) stock in Tg is listed for each blue carbon system (MN = mangroves, SG = seagrass, SM = saltmarsh) in each sub-region. The underlying blue carbon map shows the distribution of mangroves (red), saltmarsh (yellow), and seagrass (blue) (used with permission from Chmura and Short, 2015).

Of the GoM blue carbon systems studied, mangroves sequester the most carbon, storing nearly 200 Tg C_org over 650,482 ha (Figure 1). Seagrass is ubiquitous throughout the GoM basin, spanning over 1 million ha and storing 184 Tg C_org, Salt marshes, which are predominantly found in the northwestern quadrant of the GoM account for just under 100 Tg C_org. In addition to presenting these updated blue carbon stock estimates for the GoM, this study estimates anthropogenic impacts on GoM blue carbon storage and compares GoM vs. Atlantic shoreline blue carbon habitat stocks and extents.

Authors:
Anitra L. Thorhaug (Yale University)
Helen M. Poulos (Wesleyan University)
Jorge López-Portillo (Instituto de Ecología Mexico)
Jordan Barr (Elder Research)
Ana Laura Lara-Domínguez (Instituto de Ecología Mexico)
Tim C. Ku (Wesleyan University)
Graeme P.Berlyn (Yale University

Rapid warming and salinity changes mask acidification in Gulf of Maine waters

Posted by mmaheigan

· Wednesday, February 20th, 2019

Why don’t we see ocean acidification in over a decade of high-frequency observations in the Gulf of Maine? The answer lies in a recent decade of changes that raised sea surface temperature and salinity, and in turn dampened the expected acidification signal and caused the saturation states of calcite minerals to increase. From 2004 to 2014, sea surface temperatures in the Gulf of Maine were higher than any observations recorded in the region over the past 150 years. This greatly impacted both CO₂ solubility and the sea surface carbonate system, as detailed in a recent paper in Biogeochemistry.

Over the 34 years of the time-series, the recent event is extreme, but interannual and decadal salinity and temperature variability also influenced carbonate system parameters, which makes it difficult to isolate and quantify an anthropogenic ocean acidification signal, especially if relying on shorter-term observations (Figure 1).

Figure 1: Modeled Ω_Aragonite (top panel) and pH (bottom panel) anomalies relative to monthly 2004 data. The red lines show trends prior to and after 2004, after which warming accelerated.

For those with a stake in profiting from or managing extractive resources that are susceptible to ocean acidification such as commercially important lobster and bivalves, understanding how ecosystems will be affected is critical. These analyses clearly demonstrate how physical processes can either accelerate or mitigate ocean carbonate system changes, thus confounding the detection of ocean acidification that is expected from increasing atmospheric carbon dioxide. To assess whether an ecosystem or species is at risk or aided by such processes, it is important to observe, understand, and be able to model all sources of carbonate system variability.

Authors:
Joe Salisbury and Bror Jönsson (Both at Ocean Processes Analysis Laboratory, University of New Hampshire)

Biological and physical controls on estuarine nitrous oxide emissions

Posted by mmaheigan

· Tuesday, February 5th, 2019

Nitrous oxide (N₂O) is a potent greenhouse gas with rising atmospheric concentrations. Atmospheric emissions of N₂O are predicted to increase with continued anthropogenic perturbation of the nitrogen cycle, yet the magnitude of these emissions is uncertain, particularly in coastal systems where N₂O fluxes are poorly constrained. How do N₂O emissions from a eutrophic estuary vary in space and time?

Figure 1: Depth profiles of nitrous oxide (N₂O) (circles), salinity (dashed line), and dissolved oxygen (solid line) in the Chesapeake Bay at three stations. Solid circles indicate oversaturation of N₂O with respect to equilibrium with the atmosphere, and open circles indicate undersaturation.

In a recent publication in Estuaries and Coasts, Laperriere et al. (2018) examined how physical and biological processes influence the distribution of N₂O in Chesapeake Bay using dissolved gas measurements (N₂O and N₂/Ar) and stable isotope tracer incubations. During stratified summer conditions, the mesohaline region of the Chesapeake Bay was always a source of N₂O to the atmosphere. The highest N₂O concentrations occurred in the pycnocline at the interface between reducing bottom waters and oxygenated surface waters (Figure 1). Vertical mixing of surface waters across the pycnocline caused elevated rates of ammonia oxidation, a biological source of N₂O, and resulted in the accumulation of nitrite (NO₂^–) below the pycnocline. During periods of weak mixing, ammonia oxidation rates and N₂O concentrations were lower, and low dissolved oxygen concentrations below the pycnocline set the stage for N₂O consumption via denitrification (Figure 1). The interplay between biological and physical processes controlling fluctuations in N₂O concentration was examined using a mass balance approach. Mass balance estimates indicated that both biological processes and physical transport contribute to local changes in N₂O concentration. The authors suggest that the fate of N₂O during stratified summer conditions is governed by vertical mixing across the pycnocline, controlling whether N₂O is released to the atmosphere or consumed at depth.

Authors:
Sarah M. Laperriere (University of California, Santa Barbara)
Nicholas J. Nidzieko (University of California, Santa Barbara)
Rebecca J. Fox (Washington College)
Alexander W. Fisher (University of California, Santa Barbara)
Alyson E. Santoro (University of California, Santa Barbara)

Evidence against an Arctic Ocean methane bomb

Posted by mmaheigan

· Tuesday, February 5th, 2019

Gas hydrates are an ice-like storehouse of the greenhouse gas methane found in continental margins of the world ocean. Warming waters can cause hydrates to decompose and release ancient methane to overlying sediment and waters. The continental shelves of the Arctic Ocean have been thought of as “ground zero” for the potential release of methane from hydrates, since the Arctic is warming rapidly and hydrates are found at relatively shallow water depths there. Another potential ancient methane input to Arctic shelf waters is the methane produced by microorganisms from the gradual thawing of permafrost carbon within seafloor sediment and/or transported to the shelf from terrestrial permafrost via rivers. But, can large stores of ancient-sourced methane reach surface waters and enter the atmosphere, contributing to greenhouse warming?

Figure caption: Map showing the fraction of methane in each surface water sample that was derived from ancient hydrate or permafrost, on a scale from 0 (modern, 0% ancient; indigo) to 1 (100% ancient; yellow). While some of the near-shore surface methane samples have a significant (~50%) ancient component, in waters deeper than 20 m, the surface water methane was mostly (90-95%) derived from modern sources.

To answer this question and understand the role of these ancient sources of methane (hydrates and permafrost), the authors of a 2018 study in Science Advances measured the natural abundance of radiocarbon (¹⁴C) in dissolved methane in the shallow shelf waters of the Alaskan Arctic Ocean (U.S. Beaufort Sea); methane derived from ancient sources has little to no measurable ¹⁴C because of radioactive decay over time. The ¹⁴C-methane results show that ancient sources are contributing methane to the study area’s waters, as the authors predicted. However, ancient methane emitted to seawater can be consumed by microorganisms or transported away by currents before reaching the atmosphere, though these mechanisms have not been known to be effective at removing methane in waters <100 m. This study revealed that these removal processes are surprisingly efficient in shallow shelf waters, especially at the study area’s deepest stations of 30 and 40 m depth, where only ~10% of the methane in surface waters was derived from ancient sources. These results add to a growing body of evidence against the likelihood of a large methane emission to the atmosphere occurring from ancient sources like hydrates, since the authors expect that methane removal processes in the water column are much more efficient in waters 100s of meters deep, where the bulk of the hydrate reservoir resides.

Authors:
K.J. Sparrow (University of Rochester; current address: Florida State University)
J.D. Kessler (University of Rochester)
J.R. Southon (University of California Irvine)
Garcia-Tigreros (University of Rochester)
K.M. Schreiner (University of Minnesota Duluth)
C.D. Ruppel (USGS)
J.B. Miller (University of Colorado Boulder; NOAA)
S.J. Lehman (University of Colorado Boulder)
Xu (University of California Irvine)

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.

Author Archive for mmaheigan – Page 23

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