Ocean acidification and Decadal Alkalinity Variability in the North Pacific

Chair: James Orr

Jessica N. Cross(1,2), Brendan R. Carter(2), Samantha A. Siedlecki(3), Simone R. Alin(1), Andrew G. Dickson(4), Richard A. Feely(1), Jeremy T. Mathis(5), Richard H. Wanninhkof(6), Alison M. Macdonald(7), Sabine Mecking(8), and Lynne D. Talley(4)

1 NOAA Pacific Marine Environmental Laboratory, Seattle, WA, 98115, USA
2 Cooperative Institute for Alaska Research, University of Alaska Fairbanks, Fairbanks, AK, 99775, USA
3 Joint Institute for the Study of Atmosphere and Ocean, University of Washington, Seattle, 98195, USA
4 University of California San Diego, La Jolla, CA, 92093, USA
5 NOAA Arctic Research Program, Ocean and Atmospheric Research, Silver Spring, MD, 20910, USA
6 Atlantic Oceanographic and Meteorological Laboratory, Miami, FL, 33149, USA
7 Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA
8 Applied Physics Laboratory, University of Washington, Seattle, WA, 98105, USA


Background

Recent observations of acidification-driven shoaling of the calcium carbonate saturation horizon in the North Pacific have prompted new interest in carbonate cycling in this region, particularly related to impacts on biogenic calcification and dissolution at the surface layer. Some estimates project that after several decades of declining pH values, the impacts of ocean acidification on alkalinity cycling are now beginning to emerge.

Methods
Total alkalinity concentrations along a meridional transect of the North Pacific (WOCE, CLIVAR, and US GO-SHIP line P16N; 152 °W) have been collected over a period of three decades, allowing for decadal snapshots of alkalinity cycling. Calculations estimating the impact of other ocean chemical processes, such as decadal oscillations, mixing and denitrification, allow for the exploration of potential impacts of acidification on alkalinity cycling occurring in the background.

Findings
The largest source of variability in alkalinity concentrations is related to North Pacific circulation, particularly in the surface mixed layer. Precise normalization of these data reveal some small spatial and temporal variability in the background (on the order of +10 µmol kg-1). A stronger buildup of alkalinity was observed near the subsurface layer of the subarctic boundary (on the order of +20 µmol kg-1).

Conclusions
The subarctic boundary is a region of known sediment, calcium carbonate, and silicate export, indicating that the ocean build-up of alkalinity over time could correspond to increases in the rate of dissolution processes. The greatest build-up of alkalinity was also found near the aragonite and calcite saturation horizons. While the variations observed here are small, these correlations indicate that ocean acidification could be contributing to enhanced carbonate dissolution in this area. Further observation and monitoring in this area will be critical to understanding how these processes unfold over the coming decades.

David vs. Goliath: the Importance of Local Processes in Mediating the ocean acidification Signal in Shelf Seas

Chair: Samantha Siedlecki

Yuri Artioli (1)*, Sarah Wakelin (2), Jason Holt (2), Susan Kay (1), John Bruun (1), Jeremy Blackford (1)

1 Plymouth Marine Laboratory, Plymouth, PL13DF, United Kingdom
2 National Oceanographic Centre, Liverpool, L35DA, United Kingdom

Background
Changes in the carbonate chemistry due to the increasing concentration of atmospheric CO2 are locally mediated by other drivers, including biological ones like primary production. Although the atmospheric driver is clearly the major one, in productive regions the magnitude of Ocean Acidification and the variability of the carbonate chemistry are significantly affected by the local drivers. Impacts of Ocean Acidification on biogeochemical processes can further increase the importance of the local drivers.

Methods
The coupled marine ecosystem model NEMO-ERSEM has been run to project the state of the North Western European Shelf until 2100 under the IPCC RCP 8.5 scenario. Outcomes have been analysed applying Box-Jenkins time series analysis methods to emphasise the structure of the Ocean Acidification signal, in particular the differences in trend and seasonal cycle. The tool has been applied at different spatial and temporal scales to study the impact of resolution on the assessment of Ocean Acidification, and to different variables to investigate what drives the variability in response to Ocean Acidification.

Findings
Significant spatial differences in the global trend of surface pH have been observed, with areas with higher net primary production projected to experience a lower acidification (up to 30% lower). Changes in the pH seasonal cycle have also been observed, with larger amplitude (up to 40%) in areas where primary production is projected to increase, and a shift in phase correlated to changes in plankton phenology. These changes are exacerbated when a positive feedback of Ocean Acidification on primary production is considered in the model.

Conclusions
This study highlights that local processes are important to assess the signal of Ocean Acidification and its impact on organisms, since this is driven by local condition of the carbonate chemistry.

Time of emergence for acidification and de-oxygenation in a water mass framework in the North Pacific

Chair: James Orr

Rodgers, Keith B. (1), Coronado, Maricela (2), Schlunegger, Sarah (1), Frölicher, Thomas L. (3), Frenger, Ivy (1), Ishii, Masao (4), Sasano, Daisuke (4)
1 Princeton University, AOS Program Princeton, NJ, 08544, USA
2 Princeton University, Princeton, NJ, 08540, USA
3 Environmental Physics, Institute of Biogeochemistry and Poluttant Dynamics, ETH, Zürich, Switzerland
4 Oceanography and Geochemistry Research Department, Meteorological Research Institute, Tsukuba, Japan

Background
Potential marine ecosystem stressors, such as acidification and de-oxygenation, are expected to impact ocean biology over the course of the 21 st century. Detection of acidification and de-oxygenation is complicated by elevated natural background variability of the climate system. This presents a challenge for inferring secular trends from repeat hydrographic measurements.

Methods
We consider a large initial-condition ensemble suite of simulations with GFDL’s Earth system model ESM2M over 1950-2100. The ensemble approach provides a means to deconvolve natural variability and the forced secular trend, and this is approached in a water mass framework (Frölicher et al., 2009). The initial analysis is applied to interpret the repeat hydrographic section along 165°E reported by Sasano et al. (2015) for the emergence of oxygen and acidification trends. Emergence in this framework is defined as when the anthropogenic signal exceeds the noise level of natural variability.

Findings
The emergence of anthropogenic trends in acidification ( arag) along 165°E between 0°N and 50°N emerge sooner and with greater confidence than do trends in ocean interior O2 over a 30-year time frame across the major thermocline water masses of the North Pacific. The prior emergence of arag relative to O2 within the ocean interior over the historical period largely reflects the difference in the atmospheric boundary conditions for these two fields.

Conclusions
Detection of trends in ocean interior acidification and de-oxygenation is considered in a water mass framework for the North Pacific. The results indicate that acidification trends emerge prior to de-oxygenation, while simultaneously supporting the arguments of Sasano et al. (2015) that natural decadal variability is substantial and complicates identification of trends on decadal timescales. Trend detection is particularly complicated near gyre boundaries. The results for 165°E are shown to be more generally representative of the North Pacific for the ESM output.

Comparing the Sensitivity and Distribution of the Southern Ocean Surface Water pH to Mixing in a High Resolution Coupled Climate Model and the CMIP5 Earth System Models

Chair: Kumiko Azetsu-Scott

Joellen Russell (1)

1 University of Arizona, Tucson, AZ 85721, USA

We examine the relationships between the distribution of pH in the surface waters of the Southern Ocean, the air/sea flux of carbon dioxide, and the upwelling of old, low pH water. Efforts to determine the relative effects of these two processes have been hampered by the enormous computational resources required to simulate ocean biogeochemistry in eddy-resolving coupled climate models. Using observationally-based metrics and the Southern Ocean State Estimate, we first assess the relationship between upwelling and uptake on the distribution of surface pH in the CMIP5 ESM historical simulations. We then compare these moderate resolution simulations to that from a high resolution eddy-resolving coupled climate model (GFDL-CM2.6) simulation. We then assess the simulated surface pH changes in the models in response to an atmospheric CO2 doubling scenario.

Climatological distribution of aragonite and calcite saturation states in the global oceans

Chair: Joellen Russell

Li-Qing Jiang (1)*, Richard A. Feely (2), and Krisa M. Arzayus (3)

1 Cooperative Institute for Climate and Satellites, Earth System Science Interdisciplinary Center, University of Maryland, College Park, Maryland, USA.
2 Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington, USA.
3 National Centers for Environmental Information, National Oceanic and Atmospheric Administration, Silver Spring, Maryland, USA.

Background
Surface distribution of aragonite and calcite saturation states was reported by Feely et al. [2009a] and Takahashi et al. [2014]. The saturation horizons (depths where saturation state is equal to 1) in both spatial and vertical dimensions were presented by Feely et al. [2009b] using data collected before 1999. Over the past decade, ocean station data with multiple carbon parameters measured have nearly doubled. The richer data could enable a better depiction of the global distribution of aragonite and calcite saturation states.

Methods
We used data from the Global Ocean Data Analysis Project (GLODAP), the Carbon Dioxide in the Atlantic Ocean (CARINA), the Pacific Ocean Interior Carbon (PACIFICA), and some recent cruise data sets to re-examine the distribution of aragonite and calcite saturation state. The saturation states were calculated from in-situ temperature, pressure, salinity, dissolved inorganic carbon (DIC), total alkalinity (TA), silicate and phosphate.

Findings
Surface Ωarag in the open ocean was always supersaturated (Ω>1), ranging between 1.1 and 4.2. It was above 2.0 (2.0-4.2) between 40°N and 40°S, but decreased towards higher latitude to below 1.5 in polar areas. Vertically, Ωarag was highest in the surface mixed layer (SML). Seasonally, surface Ωarag above 30° latitudes was about 0.06 to 0.55 higher during warmer months than during colder months in the open-ocean waters of both hemispheres. Decadal changes of Ωarag in the Atlantic and Pacific Oceans showed that Ωarag in waters shallower than 100 m depth decreased by 0.10±0.09 (–0.40±0.37% yr–1) on average from the decade spanning 1989-1998 to the decade spanning 1998-2010.

Conclusions
The study identifies the Arctic and Antarctic oceans, and the upwelling ocean waters off the west coasts of North America, South America and Africa as regions that are especially vulnerable to ocean acidification. We also discuss the mechanisms controlling global distribution of aragonite and calcite, and examine its decadal and seasonal changes.

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