Scientific drilling and the evolution of the earth system: climate, biota, biogeochemistry and extreme systems

A US National Science Foundation-funded workshop occurred 17–19 May 2013 at the University of Oklahoma to stimulate research using continental scientific drilling to explore earth’s sedimentary, paleobiological and biogeochemical record. Participants submitted 3-page “pre-proposals” to highlight projects that envisioned using drill-core studies to address scientific issues in paleobiology, paleoclimatology, stratigraphy and biogeochemistry, and to identify locations where key questions can best be addressed. The workshop was also intended to encourage US scientists to take advantage of the exceptional capacity of unweathered, continuous core records to answer important questions in the history of earth’s sedimentary, biogeochemical and paleobiologic systems. Introductory talks on drilling and coring methods, plus best practices in core handling and curation, opened the workshop to enable all to understand the opportunities and challenges presented by scientific drilling. Participants worked in thematic breakout sessions to consider questions to be addressed using drill cores related to glacial–interglacial and icehouse–greenhouse transitions, records of evolutionary events and extinctions, records of major biogeochemical events in the oceans, reorganization of earth’s atmosphere, Lagerstätte and exceptional fossil biota, records of vegetation–landscape change, and special sampling requirements, contamination, and coring tool concerns for paleobiology, geochemistry, geochronology, and stratigraphy–sedimentology studies. Closing discussions at the workshop focused on the role drilling can play in studying overarching science questions about the evolution of the earth system. The key theme, holding the most impact in terms of societal relevance, is understanding how climate transitions have driven biotic change, and the role of pristine, stratigraphically continuous cores in advancing our understanding of this linkage. Scientific drilling, and particularly drilling applied to continental targets, provides unique opportunities to obtain continuous and unaltered material for increasingly sophisticated analyses, tapping the entire geologic record (extending through the Archean), and probing the full dynamic range of climate change and its impact on biotic history. 1 Scientific rationale Over the past decade, numerous workshops and study groups within sedimentary geology, paleoclimatology, and paleobiology have repeatedly highlighted the importance of continental scientific drilling to address important science questions in climate and linked earth systems (e.g., the US National Science Foundation (NSF)-supported GeoSystems, DETELON, and Transitions workshops, and the US National Research Council (NRC) reports on Deep-Time Climate and Climate and Human Evolution, Soreghan et al., 2003, 2005; Montanez and Soreghan, 2006; Bottjer and Erwin, 2010; NRC, 2010, 2011; Parrish, 2012). In recognition of this sustained surge of interest, a workshop was held on 17–19 May 2013 at the University of Oklahoma to identify high-priority targets for scientific drilling aimed at assessing critical questions related to paleoclimate, paleobiology, and extreme events in earth’s history. The objectives of the workshop were to (1) develop a community of researchers interested in using scientific drilling for stratigraphic targets to answer questions about earth system evolution, (2) identify topics and drilling targets of broad scientific interest, and Published by Copernicus Publications on behalf of the IODP and the ICDP. 64 G. S. Soreghan and A. S. Cohen: Scientific drilling and the evolution of the earth system (3) offer researchers direction on how to develop compelling drilling proposals, how to evaluate and plan logistical issues related to drilling, and how to develop a funding plan for drilling strategically. The ultimate intent is to galvanize research teams to move forward with future proposals involving continental drilling. 2 Workshop format and proceedings All interested researchers were invited to submit a brief preproposal identifying a viable continental scientific drilling target to examine questions of scientific importance in the areas of paleoclimate, earth history, stratigraphy, paleoecology and/or paleobiology from any interval of earth history. The 41 participants submitted 30 pre-proposals that articulated scientific themes in earth, and evolutionary and ecological history spanning geologic time. Participants discussed the role of drill core studies in earth/life history and identified locations where critical questions can best be approached. The workshop included plenary sessions in which invited speakers addressed topics of overarching interest in scientific drilling. Additionally, principal investigators for each submitted pre-proposal presented a 5 min talk outlining the science drivers, significance, and rationale to enable all participants to grasp the vast breadth of proposed projects, which span the geological record from the oldest sedimentary rocks to modern lakes, and range geographically from pole to pole (Fig. 1 and Appendix Table A1). Through the rest of the 2.5-day workshop, breakout groups identified critical science questions in earth history that can be assessed with drill core data, and evaluated disciplinespecific drilling problems. Themes were chosen to explore earth history processes across timescales, and to consider how drilling can inform our understanding of those events. 3 Glacial–interglacial and icehouse–greenhouse transitions and biotic consequences Earth’s atmosphere reached a historic threshold in May 2013, when sustained atmospheric CO2 levels exceeded 400 ppmv for the first time in human history, and the first time in approximately 3 My (Beerling and Royer, 2011). Passing this threshold offers the opportunity to engage the public in climate transitions in earth history, and the earth system linkages accompanying these transitions, such as changes in biodiversity and ecosystems during glacial–interglacial transitions. To this end, much research has focused on, in particular, the transition from the Last Glacial Maximum (LGM) to our current interglacial, as well as other transitions within the Pleistocene (e.g., Fawcett et al., 2011; Ivory et al., 2011; Brigham-Grette et al., 2013). These studies provide important insights into earth’s climate behavior at a very highresolution (millennial to annual) scale that are critical for grasping earth’s recent climate behavior and informing deeptime studies of glacial–interglacial transitions. It is equally important, however, to understand the potential diversity of glacial–interglacial behavior throughout earth’s record. What insights can we gain into climate behavior by examining Paleozoic and Neoproterozoic glacial–interglacial and – at a broader scale – so-called “icehouse–greenhouse” transitions (e.g., Bao et al., 2008; Fike et al., 2006; Kennedy et al., 2008; Mills et al., 2011; Barham et al., 2012)? Research over the past decade has highlighted the occurrence of major biological turnovers in earth history accompanied by changes in greenhouse forcing, ocean acidification and climate change (e.g., Jiang et al., 2009; Johnston et al., 2012; Katz et al., 2008). Continental drilling can provide highresolution records of these climate and life transitions. Looking back into the period of the Cenozoic when atmospheric CO2 levels differed from today provides estimates of future climate change and ecological response. The deep past also highlights abrupt change, the information necessary to test how well climate models will predict abrupt changes in the coming centuries (Valdes, 2011). The Paleozoic and Neoproterozoic icehouse–greenhouse transitions provide particularly extreme examples that, albeit poorly constrained, span evolutionary events such as the origin of animals and terrestrial ecosystems central to our existence. 4 Records of long-term evolution events and extinctions Developing reliable records of evolution and extinction requires access to high-resolution geochronological control, and also unambiguous superposition, which is a great benefit of drill core over purely outcrop studies. Furthermore, both microand macrofossil records are critical for assessments of life transitions, so linked outcrop and coring studies are critical for questions targeting these issues. In addition, a core provides access to unweathered material critical for geochemical analyses, including organic (e.g., biomarker) and potentially fossil DNA studies, which can shed light on ecological (including catastrophic) events and evolutionary history (e.g., Clyde et al., 2013). Effective strategies for obtaining sufficient sampling across critical intervals include planning for multiple shallow cores across a series of dipping strata, or targeted coring of key intervals defined by nearby outcrop studies. Drill cores eliminate the human bias of outcrop-based studies by taking an essentially random sample of local sedimentary facies through time and providing a continuous (albeit not necessarily complete) stratigraphic record across biotic transitions. 5 Records of major biogeochemical events in the oceans The first oxygenation of the atmosphere and ocean is a critical biogeochemical event in earth history. This transition Sci. Dril., 16, 63–72, 2013 www.sci-dril.net/16/63/2013/ G. S. Soreghan and A. S. Cohen: Scientific drilling and the evolution of the earth system 65 Figure 1. Geologic timescale (above) and global map (below) listing the (enumerated) 30 drilling project pre-proposals submitted to this workshop. The gray to black bars show the various timescales of the projects proposed. Note that the upper timescale is an enlarged version of the Holocene through Neogene interval, to portray projects proposed for this “near-time” slice. See Appendix Table A1 for the titles of the proposed projects (key to the numbers in the bars in this figure). was a prerequisite to the appearance of eukaryotic organisms and ultimately to emergence of animals and their major radiation – the Cambrian explosion. Many evolution and extinction events, especially those in earth’s Precambrian record, are tied either directly or indirectly to oxygen availability. Research on Precambrian earth systems has experienced a resurgence of interest as new geochemical tools have come online to assess these events – especially various redox-sensitive metal tracers and their isotope systematics (Buick, 2007; Scott et al., 2008; Partin et al., 2013; Reinhard et al., 2013) and basin-scale reconstructions of redox state (Sperling et al., 2013a, b) that enable tracking of oceanic oxygen levels. Collectively, this work demonstrates that the view of a single large oxygenation event yields to a model of dynamic rises and falls in oxygen levels. This requires that the precision of our proxies increase and/or expand to meet the constraints placed by physiology (microbial and metazoan) in order to understand earth’s major biological transitions (Sperling et al., 2013). Improved understanding of Precambrian oxygenation events also informs views of the global ocean redox landscape during Phanerozoic anoxic events (Lyons et al., 2009). Drill-core records provide a unique and fundamental opportunity to assess timing, duration, and possible drivers of these events, owing to their capacity to provide both continuous and, critically, unweathered (unoxidized) www.sci-dril.net/16/63/2013/ Sci. Dril., 16, 63–72, 2013 66 G. S. Soreghan and A. S. Cohen: Scientific drilling and the evolution of the earth system material. Beyond the ocean, coring of paleosols sheds light on the spread of oxygenation to continental systems and a direct measure of atmospheric oxygen. Further, refined data on ancient oxygen conditions inform our view of ocean deoxygenation scenarios under recent human influences, such as lower oxygen solubility in warmer surface waters and elevated riverine nutrient delivery (e.g., Falkowski et al., 2011). Although atmospheric and oceanic oxygenation are tied directly or indirectly to many of the most fundamental biogeochemical events in earth history, other events best accessible through continuous and unweathered cores include ocean-acidification events (e.g., the Permian–Triassic and Paleocene–Eocene Thermal Maximum, as potential analogs for earth’s near-term future, e.g., Zachos et al., 2005), the C3–C4 vegetation transition, and the effect on global carbon cycling linked to the radiation of diatoms and calcareous nannoplankton more recently (Katz et al., 2005). Drill cores can also provide links between oceanic and continental records to assess the role of continental processes such as nutrient recycling during times of oceanic anoxia, for example, and to evaluate the “buffering” capacity of the oceans at times of high pCO2. 6 Core records of reorganization of earth’s atmosphere Earth’s atmosphere has evolved and reorganized through time as reflected by changes in (1) chemical composition, such as the presence and proportions of O2, CO2, and SO2, (e.g., Kump, 2008; (2) aerosol composition, including the presence and proportions of mineral dust and black carbon, and – speculatively – dimethylsulfide and O3 (Wolff et al., 2010; Diessel, 2010; Bisiaux et al., 2012); and (3) circulation, such as the position of the Intertropical Convergence Zone (ITCZ), presence and strength of monsoons, and storm intensity (e.g., Ito et al., 2001; Frappier et al., 2007). Key events in atmospheric reorganization include both Precambrian and Phanerozoic shifts in (1) O2 in response to evolutionary radiations of photosynthesizers (Canfield et al., 2007); (2) CO2 in response to biological, tectonic, and weathering drivers (Bergman et al., 2004; Berner, 2006); and (3) atmospheric circulation (e.g., shifts from zonal to monsoonal circulations associated with continental positions and mountain elevations) (e.g., Parrish, 1993; Clift et al., 2008). Proxies and indicators useful to reconstructing atmospheric composition include various approaches to measuring, for example, atmospheric CO2 such as paleosol carbonate isotopes, leaf stomatal densities, and bryophyte photosynthetic fractionation (e.g., Royer et al., 2001; Fletcher et al., 2008). Proxies and indicators exist for temperature, moisture, and transport, such as various isotopes, biomarkers, and eolian provenance indicators (e.g., Soreghan et al., 2002; Eglinton and Eglinton, 2008; Severman and Anbar, 2009; Pullen et al., 2011; Woltering et al., 2011; Zambito IV and Benison, 2013), all of which benefit from pristine material offered by drill cores. Target records include loess deposits, paleosols, lacustrine and epeiric marine strata, and facies conducive to preservation of leaf waxes and associated biomarkers (e.g., Brigham-Grette et al., 2013). Drilling of continental records is particularly needed to assess the range of responses on the continents to global change. The integration of proxy results to constrain climate models offers the potential to refine predictions for future sea level and agricultural productivity under higher levels of greenhouse gases. 7 Lagerstätte and exceptional fossil biota in cores Some continental cores preserve fossil Lagerstätten (exceptionally preserved fossil biota) of microfossils or very small macrofossils (Pearson et al., 2006; Wendler et al., 2011; Wolfe et al., 2006). Microfossil Lagerstätten in cores are especially useful for yielding estimates of true diversity (Jiménez Berrocoso et al., 2010). Cores can also provide critical sedimentary and geochemical context for Lagerstätten of larger fossils or extraordinarily preserved sedimentary deposits (e.g., varves or tidalites) that give detailed records of environmental conditions. Access to such deposits via core recovery facilitates high-resolution geochronological correlation and provides well-preserved samples with no weathering overprint. Analyses of Lagerstätten in the context of continuous core can reveal biotic sensitivities to environmental change and shed light on kill mechanisms during extinction events. As such, cores containing fossil Lagerstätten or exceptional sedimentary deposits offer key insights on the evolution of earth’s system states over time. 8 Drill core records of vegetation and landscape change through time Landscapes reflect the combined effects of physical, biological, and (since humans evolved) anthropogenic influences occurring on earth’s surface over time, and thus preserve a 4-D record of processes affecting a sedimentary basin (Driese and Nordt, 2013). Vegetation and landscape changes are identified through composition of and changes in, for example, fossil flora (e.g., leaves, wood, seeds, flowers, root traces, phytoliths, palynomorphs, charcoal, etc.), trace fossils, paleosols, thermochronological signatures, and various isotopic records. The geologic record of landscapes is best addressed by an approach that combines outcrop studies (where feasible) with drilling – the former to access records such as megaflora, and the latter to enable geochemical analyses of pristine material (e.g., Retallack and Dilcher, 2011). Outstanding issues in landscape analysis that could benefit from systematic inclusion of drill core records include (1) linking sediment sources to sinks, (2) characterizing extinct soil types and ecosystems, (3) determining spatial variability in fossil landscapes, and (4) determining uplift Sci. Dril., 16, 63–72, 2013 www.sci-dril.net/16/63/2013/ G. S. Soreghan and A. S. Cohen: Scientific drilling and the evolution of the earth system 67 histories (potentially rates and reliefs and even paleoelevation through time now accessible by detrital thermochronology). Ultimately, core analyses could enable systematic analysis of the evolution of earth’s critical zone through time. With regard to landscape analysis, even the surfaces encountered in cores – the hiatal events – provide key data for landscape reconstruction (e.g., Nordt et al., 2013; Sauer et al., 2013). Beyond the deep-time geological record of landscape change, the recent anthropogenic record offers insights into landscape changes wrought by pre-industrial human transformations, especially the commonly recognized but poorly characterized history of anthropogenic deforestation and fire use (Ruddiman, 2013). Cores may allow us to distinguish the onset timing and magnitude of human-induced fire, for example in the fire-adapted ecosystems of the African miombo or Australian woodlands.