Chemistry Climate Change Ocean Acidification Paleoceanography

Path of Corrosion: How Scientists Modeled Ancient Sea-Floor Acidity

Paper: Alexander, K., et al., 2015. Sudden spreading of corrosive bottom water during the Palaeocene–Eocene Thermal Maximum. Nature Geoscience, DOI: 10.1038/NGEO2430

With so much dire talk of climate change, did you know that our climate is actually cooler than it was in geologically ancient times? Approximately 55 million years ago, the climate was at a thermal maximum during the Palaeocene-Eocene era boundary, thus called the Palaeocene–Eocene Thermal Maximum (PETM). The PETM was a 10,000 year period of rapid warming that has been linked to a massive release of carbon to Earth’s oceanic and atmospheric system – much like our own current trend in atmospheric CO2. Thus, the PETM is of special interest because of its relevance to today’s climate change, and how a large and rapid release of carbon can impact global systems. But, how did it specifically affect the oceans, and how can we compare that to what is occurring today?

Many studies have estimated the rate and amount of carbon release in the PETM in order to compare it with modern climate change. Studying seafloor carbonate dissolution can tell us about the acidity of the ancient oceans: acidic conditions will cause carbonates to dissolve. Acidity of the ocean is highly affected by changes in the “lysocline” – the depth below which the rate of dissolution of carbonates increases dramatically – and the calcite compensation depth (CCD) – the point at which dissolution is equal to input from carbonates falling from the surface waters.

As would be expected of a thermal maximum with atmospheric CO2 levels well above 1400ppm, researchers found that the PETM was marked by an event of widespread dissolution of carbonates at ocean floors. The acidification was greater in the Atlantic and Caribbean waters, and less so in the Southern and Pacific oceans. What caused these spatial differences?

Figure 1: Spread of corrosive bottom water from the North Atlantic and subsequent dissolution of seafloor carbonate. Schematic diagram illustrating the circulation patterns described in the text. Surface currents are shown in yellow, and deep currents in purple (Alexander, 2015).
Figure 1: Spread of corrosive bottom water from the North Atlantic and subsequent dissolution of seafloor carbonate. Schematic diagram illustrating the circulation patterns described in the text. Surface currents are shown in yellow, and deep currents in purple (Alexander, 2015).

Researchers used the University of Victoria Earth System Climate Model (UVic ESCM) to investigate how these differences may have happened, and what it suggested for the mechanism of the corrosive waters. Their parameters included an initial CO2 concentration of 1,680 ppm and an instantaneous carbon release of 7,000 Gigatons of carbon. They assumed a relatively fresh Arctic Ocean before the PETM because precipitation exceeds evaporation over this region. Fresher sea water is highly corrosive because excess fresh water dilutes alkalinity and thus increases acidity. Because the fresher Arctic Ocean could exchange only with the North Atlantic, due to the geology of the sea floor at the time, the North Atlantic would be a probable place for corrosive water to form.

As expected, the model suggests that at the beginning of the dissolution event, corrosive waters accumulated in the deep North Atlantic. Thousands of years after a release of atmospheric carbon (such as that of the PETM), a warming of the deep ocean caused the North Atlantic water column to destabilize. This triggered deep-water formation and convection, causing the acidic bottom water to spill over an equatorial sill (that existed at the time) into the South Atlantic. Then, the corrosive water would spread through the Southern and Pacific oceans, diluting and losing its power of dissolution as it went, explaining the weaker dissolution record in these oceans. Researchers compared the model-simulated path of carbonate sediment dissolution (Figure 1) to estimates from the sediment records, and concluded that the model was consistent with the records.

However, the model cannot be considered absolute. During the PETM, the lysocline and CCD became shallower, resulting in extensive dissolution at both the sea floor and shallow sediment columns through chemical erosion. Because of this, the sediment records for the early PETM are small or incomplete in some areas. Absence of these records makes it difficult to fully interpret the dissolution intensity from sedimentary carbonate records. Fortunately, estimates of percent dissolution can be calculated from some Ocean Drilling Program (ODP) and Deep Sea Drilling Project (DSDP) drilling sites. The agreement between these estimates and simulated percent dissolution is generally good. Thus, the researchers say that their simulated model and mechanism for corrosive bottom water formation can account for the spatial differences in deep-water acidification during the PETM, and is a substantial advancement in understanding deep-sea environments during the PETM.

How can this be compared to our current climate change scenario? The model took into account previous estimates of atmospheric CO2 concentrations during the late Palaeocene, ranging from <400 ppm to >2,400 ppm. Our current CO2 levels just topped 400ppm, so we are a far cry from being in the same situation of the PETM. Yet, our ocean waters are undoubtedly becoming more acidic, and we are experiencing rapid releases of carbon to the atmosphere. While this palaeo-ocean model cannot be applied to our current predicament, advancements in modeling ocean acidity fluxes are important to understanding the possible near-future outcomes of rapid carbon release.

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