Analysis of pCO2 and Calcium Carbonate Saturation State Distributions in the Global Oceans

Abstract:

The marine carbon cycle greatly impacts global CO₂. With increased CO₂ production resulting in higher global CO₂ levels, understanding this cycle’s impact is critical. An integral component in the marine carbon cycle is calcium carbonate (CaCO₃). The CaCO₃ saturation state (omegaCa) determines whether CaCO₃ dissolution or precipitation is favored, where omegaCa is the in situ ion product of Ca²⁺ and CO₃^2- divided by K’sp (the apparent solubility product). Dissolution leads to higher ocean alkalinity resulting in higher pH and lower pCO₂ (and vice versa for precipitation). Thus, understanding omegaCa is crucial in analyzing the ocean’s impact on atmospheric CO₂.

Our work uses the GloDAP 2019 database alkalinity and dissolved organic carbon measurements to present omegaCa and pCO₂ distributions of the Arctic, Atlantic, Indian, and Pacific basins and global ocean. Except in the Arctic, all distributions are highly skewed, with mean values pulled from the mode due to a long, high-omega tail. Despite this, all basins exhibit mean omegaCa values of around two, with the lowest in the Pacific (1.93) and the highest in the Atlantic (2.31). The mean pCO₂ value of the overall oceans is 525 μatm, with individual basin variance down to 341 μatm in the Arctic and up to 620 μatm in the Pacific. We also present volume weighted distributions using an equal-area gridding and interpolation method, which shifts omegaCa distributions to lower values and pCO₂ distributions to higher values.

In analyzing total omegaCa and pCO₂ distributions, we gain a far better sense of what overall means actually represent and are able to delve into why certain oceans exhibit similar means yet have extreme differences in distribution range and shape. Perhaps most surprisingly, these data suggest that the global ocean is supersaturated with respect to calcite, despite the assumption that the marine CaCO₃ system is balanced. If this assumption is to hold true, it necessitates that dissolution must be occurring in supersaturated waters. These results present a new vantage point from which to interpret the chemical composition of the oceans and offer a new understanding of an integral component in the marine carbon cycle and its ability to influence global CO₂.