Physical Records of Ocean Acidification in the Geological Record

Abstract:

The geological record of ocean acidification can be extremely difficult to decipher, especially in deep time. Even though excellent work has been done to develop new geochemical proxies for acidification (either a drop in pH or carbonate saturation state (Ω) of the water), many events in the Mesozoic and Paleozoic remain poorly constrained. Even when evidence exists for a drop in pH, a slowing/cessation of carbonate deposition, or submarine dissolution of carbonate sediments, it is difficult, and often contentious, to confidently assess which features represent acidification events versus other phenomena. Here we discuss the sedimentological and paleontological records for several hypothesized acidification events: the end-Permian mass extinction, the Early Jurassic (Toarcian) Oceanic Anoxic Event, and the Paleocene-Eocene Thermal Maximum. These three events provide clear examples of the difficulty in determining whether or not acidification occurred in the rock record, especially in deeper time. In some cases, such as the end-Permian event, the sedimentological evidence of carbonate dissolution or halted production may be clear, but the timing does not align with other sites or proxies (e.g., the geochemical records of pH change). In other instances, the complex history of early authigenesis can obscure marine sedimentological records of carbonate shutdown and dissolution, making attribution of potential horizons difficult to decipher. While no one dataset or proxy seems to provide definitive evidence of acidification (at least not alone) during these deeper time events, this discussion will cover the strengths and potential issues associated with interpreting physical, paleontological, and geochemical evidence of ocean acidification in the rock record.