Carbonate Saturation Horizons in the Northwestern Hawaiian Islands: Implications for Deep-Sea Corals

Undergraduate #50
Discipline: Ecology Environmental and Earth Sciences
Subcategory: Climate Change

Ashley N. Davis - University of South Carolina
Co-Author(s): Kathryn E. F. Shamberger, Texas A&M University, College Station, TX; E. Brendan Roark, Texas A&M University, College Station, TX; Amy R. Baco, Florida State University, Tallahassee, FL; Jahna Brooks, Texas A&M University, College Station, TX; Kelci Miller, Texas A&M University, College Station, TX

Increasing anthropogenic carbon dioxide (CO2) production has led to a decrease in ocean pH, carbonate ion concentration ([CO32-]), and calcium carbonate saturation state (Ω) in a process referred to as ocean acidification. Ocean acidification negatively impacts marine calcifiers, including deep-sea corals that utilize carbonate ions to build their calcium carbonate (CaCO3) skeletons. Deep-sea reefs provide crucial habitat for commercially important fisheries and house several endemic species. Carbonate saturation horizons represent the depth in the ocean where Ω = 1, below which dissolution of CaCO3 is thermodynamically favored. Saturation horizons have been shown to be shoaling since preindustrial times due to ocean acidification. This shoaling threatens deep-sea corals and the valuable ecosystems they support because undersaturation (Ω < 1) impedes biogenic calcification and makes coral skeletons vulnerable to dissolution. Here, the aragonite and calcite saturation horizons (ASH and CSH, respectively) were examined near the locations of deep-sea coral beds at nine locations along the Northwestern Hawaiian Islands and Emperor Seamount Chain. Water samples were collected at these sites between 2014-2016 and analyzed for total alkalinity and dissolved organic carbon, in order to calculate the depth of the ASH and CSH. Consistent with previous studies of the region, these data show that the ASH and CSH deepen moving from southeast to northwest along the island chain. By comparing these data to those of the World Ocean Circulation Experiment (WOCE) from the 1990s, the Climate Variability and Predictability (CLIVAR) program from the 2000s, and the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) from the 2010s, our calculations indicate the ASH and CSH continue to shoal and that shoaling rates may have increased over the last 25 years. It is largely unknown how deep-sea corals that were previously in saturated water are now responding, or will soon respond, to undersaturation following shoaling of saturation horizons.

Funder Acknowledgement(s): Funding was provided by a NSF REU grant.

Faculty Advisor: Dr. Kathryn Shamberger, katie.shamberger@geos.tamu.edu

Role: I analyzed a majority of the water samples from the Northwestern Hawaiian Islands (NWHI) using a Versatile INstrument for the Determination of Total inorganic carbon and titration Alkalinity (VINDTA) to measure the total alkalinity (TA) and dissolved inorganic carbon (DIC). Under the guidance of my mentor, I post-processed the data, and using the CO2SYS program, I calculated the saturation states of aragonite and calcite in order to calculate the depths of the aragonite and calcite saturation horizon. I also compiled and analyzed data from three repeat hydrography programs to compare to the NWHI data, so I could look for temporal trends in the ASH and CSH depths.