April 1, 2019

Why Should We Care About Past Environments and Climates? Views from a Paleo-Detective.

Posted by Sharon Rauch

“Credit: Hannes Grobe / Wikimedia Commons”

I study Paleoceanography and Paleoclimatology – In the dictionary this field is defined as studies dealing with all aspects of understanding and reconstructing Earth’s past climate and environments – but most people have no idea what that means.  How can we study something that is not there anymore, events that happened millions of years ago, and why would anyone want to know what the climate was a decade ago let alone in centuries past? When asked what I do I explain that the work of a paleoceanographer is very much like that of a detective – we search for clues and evidence to reconstruct past events, we examine records, take fingerprints and conduct meticulous tests to further interrogate the data and check and cross-check the material witnesses. We must be careful and methodical in our investigations. Once we collect all available information, we try to put all the pieces of that puzzle together, at least those that we have access to, to provide the best solution that is consistent with all the information we gathered regarding the “crime scene”.  We can decipher what dinosaurs eat, what was the vegetation like where they lived and the temperatures in these environments. We can determine how ocean currents moved and how the chemistry to the water changed along the flow path. But like in the best detective movies and thrillers things might get complicated, you may have arrested the wrong suspect while the culprit is roaming free; New tests and technologies (like DNA analysis) may later become available and reveal new information; or some old documents are found that change the previous interpretation. So back to the drawing board in a constant effort to refine your understanding and seek the truth.

You may wonder – Why bother? Why should we care about past environments and climates? – One important motivation is so we can learn how our dynamic Earth functions and understand the various processes that control the conditions at any location and time. This is crucial if we want to be able to predict what future Earth may have in store for us and be able to plan for such changes. However, because many of the processes that make our planet Earth habitable and define the conditions in our living environment (for example mountain uplift and erosion, changes in ocean circulation or in the composition of the atmosphere) are very slow, it would take way longer than a persons’ lifetime or even that of many generations to be able to measure these processes. Moreover, the background conditions in the future may be different than at present (for example higher temperatures or no sea-ice in the Arctic) so any measurements in the present may not be representative. We can however move back in time and look at the big picture or at previous potentially representative times to figure out how Earth works so we can project and anticipate what our future may look like and possibly prevent disasters or take advantage of opportunities to ensure the well-being of people and the environment.

But this is easier said than done. Although paleoceanography/paleoclimatology is a fascinating field of research – which involves meticulous and systematic work, deductive reasoning, use of cutting-edge technologies, and a lot of creativity and imagination, it is not for the faint of heart. If you need a definite answer of right or wrong, if you need the assurance that you are correct – it is not for you. Paleoceanographers must be willing to accept that while they work tirelessly to gather the most up to date, high quality information – they may never be able to be 100% sure that their interpretation of the evidence, their solution to the mystery is the absolute truth. There are always multiple possible solutions because it is impossible to constrain all processes involved (i.e. the motives and actions of all potential suspects) since some of these have long disappeared (fled the crime scene or took a different identity) or key information is obscured or missing and will never be recovered.  But if you enjoy the process, the thrill of investigation, and open to accept new ideas, this constant journey to incrementally improve our knowledge can provide endless fun and adventures.

Here is an example of an evolving story of a data set that provided me and my colleagues many opportunities to explore, refine, and re-assess our understanding of how some major processes on Earth operate – The story of sulfur in seawater.  Sulfur is an important element and it is present in rocks, the ocean, atmosphere and all living organisms. Specifically, the relative abundance of various S containing compound (gypsum, pyrite, organic sulfur) and their isotopic composition (δ34S – a measure of the ratio of 34S isotope to the 32S isotope in a compound) in various earth reservoirs, and in particular in the ocean and in marine sediments, can shed light on how much oxygen was in the ocean and atmosphere with implications to the evolution of living organisms and the abundance of many other elements and their cycles, including carbon, nitrogen, phosphorus and many important metals. Indeed, in order to understand the coupling between these cycles throughout Earth’s history, δ34S of seawater sulfate (δ34Ssw) was measured as early as the 1960’s (Holser and Kaplan, 1966) with continuous refinement of the seawater curve (see Claypool et al., 1980). Early work utilized evaoporite deposits (gypsum and anhydrite) to reconstruct δ34Ssw but due to the sporadic occurrence of these deposits in the geological record and their susceptibility to dissolution and change after burial the temporal resolution and the errors associated with the data were quite large.

In 1998, after spending two years of long hours in the lab, I and my co-authors were able to produce the first high resolution δ34Ssw using marine barite (Paytan et al., 1998). Getting the data was hard work, but once the method was established, it mostly required time and careful analytical work. Interpreting the data was a whole other challenge. The curve (Figure 1) displayed a change of about 5‰ over a period of ~7 million years, starting around 53 Ma, an unexpected and surprising results considering the high concentrations and long residence time of sulfate in the seawater (~20 million years). Such a shift requires substantial changes in the S cycle. Using mass-balance and making some assumptions about the various processes involved, I suggested that the increase could be a result of a pronounced decrease in hydrothermal activity combined with a large increase in the rate of pyrite formation, but I also acknowledged that the system is under constrained and multiple solutions are possible. Five years later, using more sophisticated modeling coupled in collaboration with other colleagues (Kurtz et al., 2003), an alternative explanation for the rise in δ34Ssw was suggested; namely a shift in the setting of organic C and S burial form euxinic (low δ34Ssw) to an open ocean setting (higher δ34Ssw) related to sea level rise in the early Eocene.  Additional ideas were put forth by others, for example, Ogawa et al. (2009) postulated that extensive burial of pyrite in the Arctic Ocean cause the increase in δ34Ssw.  In 2012, together with Wortmann, I provided yet another possible explanation to the observed change – this time suggesting that the δ34Ssw shift can be ascribed to the dissolution of a basin scale evaporite of Neoproterozoic to Early Cambrian age with high δ34S following the India-Eurasia collision in conjunction with low seawater sulfate concentration. Finally, just last year Rennie et al., (2018) suggested that the observed change can result from a shift in pyrite deposition from shallow to deep water setting and related change in the isotopic composition of pyrite. The reason for this change was attributed to the collision of India with Eurasian and associated sea level change. I am sure the story is not over and there will be more to come, and I am very much looking forward to continuing my adventures and enjoying this rewarding journey as a paleo-detective.

Dr. Adina Paytan, University of  California, Santa Cruz