The Oceans: A Deep History

Eelco J. Rohling

Princeton University Press 2017
A book review by Danny Yee © 2018
A history of the Earth's oceans necessarily involves the broader history of the planet, since ocean currents shift with the configuration of continents and ocean chemistry changes with the composition of the atmosphere and with cycles of uplift and weathering. Rohling's focus in A Deep History is on ocean chemistry — he treats circulation patterns much more briefly — but he embeds that in the broader science and history, ranging from chemistry to plate tectonics and finding a good mix of broad picture and local detail.

Rohling begins by looking at the origins of the Earth, plate tectonics and the changing shape of ocean basins, salt and the ocean circulation, and the history of life and the carbon and oxygen cycles. Then he considers some of the driving forces: orbital and solar variations, greenhouse gases, tectonics and weathering, and impacts. The remainder of The Oceans progresses roughly chronologically, but jumps forward when details from more recent periods, with more accurate records, can help us understand the processes involved.

"Snowball Earth" episodes, most recently about 640 million years ago, may have seen even equatorial regions as cold as -20°C. They were "most likely caused by a rare confluence of three factors: weak solar output, changes in Earth's reflectivity to incoming solar radiation, and greenhouse-gas changes". Breaking out of a snowball state required colossal concentrations of atmospheric CO₂, which then remained, resulting in an abrupt transition into a "supergreenhouse" state, eventually ended by high rates of chemical weathering and CO₂ consumption.

The connection between ocean chemistry and the Ediacaran and Cambrian explosions of life is unclear, but Rohling suggests sponges may have been instrumental in ocean oxygenation and the other effects of life were dramatic. Around 540 million years ago came the development of carbonate skeletons and the transition from "Strangelove" oceans to "Neritan" ones dominated by biological carbonate deposition; "the fundamental components of modern marine food webs were set up, and much of the oceans had become oxygenated".

The end-Permian extinction event, about 250 million years ago, saw "a very fast addition of a large quantity of external carbon into the atmosphere-ocean-biosphere system", leading to "a triple whammy of intense global warming, ocean anoxia, and then severe ocean acidification". In "The Oceans on Acid" this, along with a "flash-forward" to acidification in the later PETM (Paleocene-Eocene Thermal Maximum), forms the background for an explanation of the interacting organic and inorganic carbon cycles in the ocean. One key boundary is the carbonate compensation depth, below which carbonate dissolves faster than it is rained down from above. With the development of planktonic calcifers, a key driver of the inorganic carbon cycle, the end-Permian also saw the transition to modern "Cretan" oceans.

The "Age of Reptiles", from Rohling's perspective, was "150 million years with sustained high rates of ocean spreading, intense subduction... major volcanism, and high sea levels", the result of "a single underlying mechanism: intense plate-tectonic activity". His focus here is on ocean anoxic events and the sapropels (organic-rich layers of dark sediments) they left behind. He also looks at the creation, through repeated episodes of evaporation, of "salt giants", ranging from hundreds of thousands to over a million square kilometres in area and several hundred to a few thousand metres in thickness. There are comparisons with anoxic events in the Mediterranean over the last 15 million years, and with the Mediterranean salt giant which formed over 600,000 years, starting around 6 million years ago.

Perhaps because some of it has been covered already, there are just thirty pages in "Winter is Coming" on the last 65 million years: the Paleocene-Eocene Thermal Maximum, the Early Eocene Climatic Optimum, and then a "long, variable cooling trend". The broader biological and geological history is sketched and topics treated at length include the changes in ice sheets and sea levels, the importance of CO₂ uptake and release from the oceans, and the see-saw of the North Atlantic Deep Water. As background to this Rohling touches on ice cores, fossil coral reefs, sediment cores, and various proxies for measuring temperatures and chemical concentrations.

In a brief final chapter Rohling looks at human carbon emissions and how they compare with past events, and at their effects, at ocean acidification and sea level rise and the possibility of disruptions to circulation patterns.

Rohling finds a good balance between explaining basic climate science and chemistry, probing more advanced topics, and narrating the broad history. I didn't find the background tedious and The Oceans never dragged for me, but it should be accessible to those with much less of a background in climate science.

Note: I don't know why A Deep History wasn't included in the Princeton Primers in Climate series, since the Climate and the Oceans volume of that is complementary, covering circulation rather than chemistry.

August 2018

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%T The Oceans: A Deep History
%A Rohling, Eelco J.
%I Princeton University Press
%D 2017
%O hardcover, bibliography, index
%G ISBN-13 9780691168913
%P 262pp