Forty years of ocean acidification observations (1983–2023) in the Sargasso Sea at the Bermuda Atlantic Time-series Study site

Abstract

Ocean physical and biogeochemical conditions are rapidly changing over time. Forty years of observations from 1983 to 2023 collected at the Bermuda Atlantic Time-series Study (BATS) site near Bermuda in the North Atlantic Ocean shows continuing trends of surface warming, increase in salinity, loss of dissolved oxygen (DO), increase in carbon dioxide (CO2), and ocean acidification (OA) effects. Over this period, the ocean has warmed by about +1°C, increased in salinity by +0.136, and lost DO by 12.5 µmol kg−1 or ~6%. Since the 1980s, ocean dissolved inorganic carbon (DIC), total alkalinity (TA), a tracer of anthropogenic CO2 (CTrOCA), and fugacities/partial pressures of CO2 (i.e., fCO2 and pCO2) have continued to increase substantially, with no evidence of a reduction in the rates of change over time. Contemporaneously, ocean pH has decreased by ~0.1 pH units [with ocean acidity (i.e., H+) increasing by >30%], and the saturation states of calcium carbonate minerals (Ωcalcite and Ωaragonite) have decreased. These OA indicators show that the chemical conditions for calcification have become less favorable over the past 40 years. Updating of data and trends at the BATS site show how ocean chemistry of the 2020s is now outside the range observed in the 1980s, and how essential these data are for predicting the response of ocean chemistry and marine ecosystems to future shifting earth and ocean conditions.

Quantifying Drivers of Seasonal and Interannual Variability of Dissolved Oxygen in the Canada Basin Mixed Layer

“…∼20-30 μmol kg 1 sampled by ITPs in this study). This North Atlantic seasonal cycle is characterized by high wintertime O 2 due to high primary production rates, and low summertime O 2 resulting from lower primary productivity, atmospheric exchange and solar warming (Bates & Johnson, 2023). Similar to the decreasing trend of mixed-layer O 2 in wintertime found in this study (approximately 1.14 μmol kg 1 yr 1 ), long-term decreasing trends in upper ocean O 2 concentrations have been observed globally (e.g., trends of around 0.31 μmol kg 1 yr 1 in the North Atlantic over 1983, Bates & Johnson, 2023.…”

“…This North Atlantic seasonal cycle is characterized by high wintertime O 2 due to high primary production rates, and low summertime O 2 resulting from lower primary productivity, atmospheric exchange and solar warming (Bates & Johnson, 2023). Similar to the decreasing trend of mixed-layer O 2 in wintertime found in this study (approximately 1.14 μmol kg 1 yr 1 ), long-term decreasing trends in upper ocean O 2 concentrations have been observed globally (e.g., trends of around 0.31 μmol kg 1 yr 1 in the North Atlantic over 1983, Bates & Johnson, 2023. Unlike the findings here for the Canada Basin mixed layer, O 2 reductions in mid-latitude regions have been attributed primarily to decreased solubility, reduced ventilation, and increased microbial respiration, all resulting from ocean warming (Breitburg et al, 2018;Helm et al, 2011;Schmidtko et al, 2017).…”