My research interests are broad, including Earth system history, geobiology and biogeochemical cycling, and (paleo)climate and environmental change on all time scales ranging from human to geological. As a result, I consider myself a “paleobiogeochemist”, an activity in which I employ methods of chemical oceanography, geo(micro)biology, molecular paleontology, analytical geochemistry, and microscopy to address a wide variety of research questions involving a wide variety of time scales. Similarly, because of the breadth of my interests, I have collaborated with a wide variety of other scientists, from stratigraphers to microbial ecologists as well as other paleoclimatologists and biogeochemists of various types.
Throughout my research, my approach has been analysis of sediments or sedimentary rocks for chemical tracers of past climate and environmental change. The primary analytical tools I have applied have been organic and stable isotope geochemistry, especially biomarker and molecular isotopic studies. In addition, I have utilized inorganic geochemistry and other approaches to expand the interpretations that could be made. In addition, early in my graduate studies I realized that in order to be able to interpret the paleo data I was producing, I also study and understand how processes work in modern systems, what sort of chemical tracers would be left behind, and how those tracers would be modified during diagenesis. A portion of my research program therefore has evolved to include the study of modern systems, and I enjoy the opportunities to carry out research at many points in space and time to study different biogeochemical processes, the tracers they leave in sedimentary systems, and the climate and environmental changes that can be reconstructed by applying these tracers in ancient systems. This research field work is international in scope including Africa, Central America and South America. Below, I highlight some of the research directions that I have been pursuing, and where I see my research developing in the future.
Biogeochemical proxy development
During the last glacial period and deglaciation, global environmental conditions shifted rapidly between extreme, but stable, states. Millennial-scale events, first identified in high-latitude Greenland ice cores, record rapid shifts between stable warm and cold conditions that took place on the scale of a human lifetime. Understanding the mechanisms that caused such rapid global climate changes in the past is one of the key problems in paleoclimate research, and critical for predicting the impact of future climate changes on humans worldwide. Temperature is one of the most critical climate parameters to understand, yet remains difficult to reconstruct.
The Earth’s surface temperature has a tremendous impact on other climate factors, such as hydrological variability. Furthermore, the recent trend of global warming illustrates our need to understand temperature, and its influence on climate change for future generations. For example, according to a recent report by the Intergovernmental Panel on Climate Change that summarized many scientific studies, the Earth warmed by 0.6 ºC during the 20th century, and is projected to warm by an additional ~2-6 ºC during the 21st century. In addition, a recent study by the National Research Council (NRC) of the National Academies concluded that the recent warming observed in many records is real, but that there is less confidence in temperature reconstructions prior to A.D. 1600, and much less prior to A.D. 900. The primary reason for a lack of confidence in temperature reconstructions, according to the NRC committee, is the relative scarcity of precisely dated proxy evidence.
Paleotemperature is difficult to reconstruct in continental systems, particularly in paleoarchives of deposited materials such as lake sediments. Recent work by my research group among others has highlighted the potential for new, microbial lipid based proxies for temperature to be applied in lake sediments. A new molecular temperature proxy based on the distribution of specific membrane lipids derived from single celled microbes, Thaumarchaeota, living ubiquitously in the oceans. This proxy, called the TEX86, is based on the observation that these archaea change the distribution of lipids in their cell membrane as a function of growth temperature. My research group made the first measurements of these compounds in lake systems, subsequently demonstrating that the TEX86 proxy is applicable in some lake systems (Powers et al., 2004). We also produced the longest, most high temporal resolution paleotemperature record from tropical Africa using TEX86 (Powers et al., 2005; Woltering et al., 2011). Further research by my group demonstrated that the TEX86 proxy is problematic in some lakes, especially smaller inland lakes (Powers et al., 2010).
More recently, very similar bacterial membrane lipids have been identified widespread in soils and peats, and they also seem to have a distribution that is controlled at least in part by temperature, which was quantified in the MBT index (e.g. Weijers et al., 2009). My research group has identified these compounds widespread in lake sediments (Blaga et al., 2010; Powers et al., 2010; Weijers et al., 2009, 2011). We also applied the MBT paleotemperature proxy to a sediment core from the Valles Caldera core in northern New Mexico, where we produced a temperature record spanning two full glacial/interglacial periods (550,000 to 350,000 years before present; Fawcett et al., 2011). Other studies, however, have demonstrated that the MBT proxy in lakes typically produces temperatures that are colder than instrumental mean annual air temperature, and that other environmental factors influence the fidelity with which the distribution of these compounds reflects the growth temperature of the bacteria producing them. Thus, the studies to date all suggest the potential to utilize these bacterial membrane lipids preserved in lake sediments as a continental paleotemperature proxy, which could be a significant step forward in continental paleoclimate studies. However, many questions remain to be answered before we can confidently apply these proxies to temperature reconstructions. Among these, one of the most significant is the bacterial source of the branched GDGTs. At present, we do not even know which bacteria synthesize branched GDGTs, though some broad suggestions have been made, including that the bacteria are likely heterotrophic.
Ongoing research by my research group is aimed at gaining a better understanding of these proxies for paleotemperature (proposal to NSF planned for Fall 2015). Appropriate application of rigorously investigated proxies such as the TEX86 and MBT provides the best information about paleotemperature estimates currently available on timescales beyond the existence if instrumental records, but the use of such proxies is only valid if we fully understand all of the caveats associated with them and apply them appropriately.
Global climate and environmental change
The questions driving research in Quaternary climate, the period of geological time that includes the recent ice ages and interglacial climatic periods, and environmental change are important not only for our understanding of how the Earth System has changed in the past, but also because this time period represents the closest analog for what we will be seeing in the near future. My research has gravitated towards understanding climate and environmental change in low latitude systems, in part because they have not been as well studied as high latitudes, but also because I find the questions and teleconnections at the low latitudes intriguing. At low latitudes, temperature changes are measureable, e.g. on glacial/interglacial timescales, but they are quite muted compared to high latitudes. In contrast, significant variability in the hydrological cycle is observed, often resulting in major shifts of arid zones associated with global climate change. Such changes in temperature and hydrology can have a significant impact on both terrestrial and aquatic biota, which can in turn lead to feedbacks to the climate system if the response of biota is great enough, as is predicted for (catastrophic) drying of the Amazon, for example. My own research has focused recently on using molecular biomarkers to reconstruct temperature quantitatively using the developing TEX86 and MBT/CBT paleotemperature proxies described above, along with molecular isotopic tools such as the carbon and hydrogen isotope composition of plant leaf waxes to reconstruct relative aridity.
In Africa, my group has identified numerous significant changes in temperature over the past 70,000 years (Woltering et al., 2011), including a warming of 3.5ºC from the Last Glacial Maximum to present (Powers et al., 2005), but also major changes just during the last 10,000 years, including a temperature shift of 2-4ºC that occurred at the end of the African Humid Period ~5,000 years ago – a time when temperatures of Lake Turkana were in fact warmer than they are today (Berke et al., 2012a). Furthermore, we have identified a substantial increase in the temperature of Lake Malawi surface waters of ~2ºC over the past 100 years, most likely attributable to anthropogenic influences (Powers et al., 2011). These changes in climate are associated with major shifts in rainfall as well (Berke et al., 2012b, 2014). Furthermore, we have identified shifts in both the algal communities of Lake Malawi (Castañeda et al., 2009a; 2011) and in the major terrestrial vegetation in southern East Africa (Castañeda et al., 2009b) that can be attributed to these changes in climate. Both of these biotic responses, and much of the temperature and hydrological variability we have identified in the region, appear to be related to migration of the InterTropical Convergence Zone (ITCZ). We have also identified teleconnections to high latitude ice sheets, which can impact aridity in East Africa through feedbacks in the global climate system that influence ITCZ migration (Castañeda et al., 2007).
Such feedbacks highlight the interesting and complex nature of attempting to understand low latitude climate change. Many different climate systems and interactions are relevant to these paleoclimate studies, such as ENSO, monsoons, the North Atlantic Oscillation, and the Pacific Decadal Oscillation, not to mention ice sheets, insolation, precession, obliquity, eccentricity and other processes that influence climate on longer time scales. Given the global population distribution today, this region is especially important to understand and model accurately, allowing predictions to be confidently made with respect to low latitude climate and climate change. These processes are all important on different timescales, and in order to really understand how they interact with each other to influence climate and environmental change globally, we need to develop more high resolution records of past changes. The tropics have fewer records at present than the higher latitudes, which greatly weakens our understanding of the role of the tropics in global change.
My continuing research directly addresses this critical need, focusing on reconstruction of temperature, hydrology, and biotic change throughout the Quaternary. In particular, my research in climate and environmental changes recorded in lacustrine deposits in southwestern North America (both the US and Mexico) and South America is expanding. Southwestern North America is projected to become more arid in the coming decades and centuries in response to ongoing climate change; these anthropogenic changes will be superimposed on natural variability in climate systems—the Mexican Monsoon, ITCZ migration, and storm tracks of winter Westerlies. My research group identified geochemical evidence of extended megadroughts in the Valles Caldera, New Mexico during Pleistocene interglacial periods, demonstrating that during warmer periods in this region the monsoon system was altered, making the region even dryer than it is currently (Fawcett et al., 2011). Along with collaborators, we are continuing this investigation into the paleoclimate of southwestern North America with a new sediment core recovered from Stoneman Lake, Arizona in October 2014 spanning nearly the past million years (proposal to NSF planned for October 2015). In addition, I am part of an international collaborative research effort to take a long sediment core from Lake Chalco, Mexico, in January/February 2016, which we expect to span up to 700,000 years (funded by ICDP, with additional funding pending from NSF). The location of Lake Chalco immediately adjacent to Mexico City gives this climate record particular societal relevance. A long and detailed climate record from central Mexico will enhance our understanding of the mid-latitude to tropical linkages in North American climate. Interplay between the Pacific and Atlantic is stronger in Chalco than at other sites, and its high altitude location is typical of much of southwestern North America. This will be among the longest archives of climate, environment, and biota from North America, from a region presently lacking such records prior to the Last Glacial Maximum (Brown et al., 2012).
Biogeochemistry of modern (aquatic) systems
My interest in the biogeochemistry of modern systems, and microbial geochemistry, was ignited when I was trying to reconstruct different aspects of the paleoenvironment, including redox conditions in the Devonian Appalachian Basin and carbon isotope biogeochemistry in the Cariaco Basin, for which we had insufficient understanding of the processes occurring in modern systems to apply tracers with confidence. Following this realization, I took on more modern research projects, including a postdoc studying the anaerobic oxidation of methane in active mud volcanoes of the Eastern Mediterranean. This particular process leaves a very clear tracer in sediments, specific microbial biomarkers with severely 13C-depleted carbon isotope values (Werne et al., 2002, 2004, 2005).
My research in modern systems has continued as a University of Pittsburgh faculty member. The two main foci of study in this area have been the ecology and biogeochemistry of aquatic archaea and the study of carbon cycling using radiocarbon to identify sources, sinks, and cycling rates, both in Lake Superior. The carbon cycling study, carried out in collaboration with scientists at the Large Lakes Observatory, demonstrated that the different pools of carbon in Lake Superior cycle on very different time scales, ranging from a residence time of ~3 years for dissolved inorganic carbon to ~60 years for dissolved organic carbon, and even longer for particulate organic carbon (Zigah et al., 2011). This comparatively long residence time of particulate organic carbon is most likely due to a subsidy of pre-aged organic carbon from the surrounding terrestrial watershed and/or sediment resuspension (Zigah et al., 2012). Further study demonstrated that the food web in Lake Superior is largely supported by recent in-lake primary production utilizing the dissolved inorganic pool, rather than heterotrophic production utilizing the pre-aged terrestrial carbon (Zigah et al., 2012), though there were some exceptions to this rule, for example the benthic amphipod Diporeia has a unique radiocarbon signature suggesting it feeds predominantly on aged sedimentary organic carbon (Kruger et al., 2015).
Another major focus of my research in modern systems for the past several years has been the biogeochemistry and ecology of aquatic Thaumarchaeota, microbes that have recently been identified living throughout the world’s oceans and lakes. These microbes synthesize the biomarkers utilized in the TEX86 paleotemperature proxy, but very little is actually known about them – indeed at present only one species of this phylum of archaea is in pure culture anywhere in the world. My group, in collaboration with microbial ecologists, has been studying these intriguing organisms, which may be important in the aquatic nitrogen cycle as well as useful for paleotemperature studies, primarily in Lakes Superior and Malawi. We have identified aspects of their growth habitat that may bias the temperatures recorded by the TEX86, at least in some systems, with significant implications for reconstructing temperatures in paleosystems (Woltering et al., 2012).
I am continuing to study these and other important biogeochemical processes in aquatic systems. For example, in numerous lakes, including Lake Malawi as well as Lake Erie, cyanobacterial blooms are becoming more common as the lake warms. In addition to problems associated with toxins produced by some species of cyanobacteria, this process may lead to a decrease in aquatic productivity propagating up the food chain and impacting fisheries. Such occurrences are problematic in Lake Erie, but can be devastating for countries like Malawi that depend on such systems for much of their dietary protein, particularly as the planet warms. Indeed, my research shows the potential for collapse of the Lake Malawi algal ecosystem during warm periods in the past (Castañeda et al., 2011), making our study of the biogeochemistry and ecology of modern Lake Malawi particularly relevant. This impact is not only important in Malawi, of course, but in similar systems worldwide, particularly in developing regions. My continuing research in this area will address important questions related to the influence of climate change on aquatic ecosystems, and how that may impact human activities and development.
I am currently working on a proposal (in collaboration with Dr. Emily Elliott, planned for submission to NSF in February 2016) that exploits the overlap in our expertise to utilize compound specific nitrogen isotope analysis to investigate nitrogen cycling and cyanobacterial productivity in lake systems. We will be proposing to study Lake Superior and Lake Erie initially as the pristine, low-productivity and impacted high-productivity end-members of the Laurentian Great Lakes, respectively, to establish the potential of this new approach. Once proven viable, we will be able to address questions in a number of other systems, including Lake Malawi, which are more remote and logistically difficult to study.
Biogeochemistry of organic sulfur
I am also interested in the biogeochemistry of organic sulfur. Organic sulfur is the second largest pool of reduced sulfur in sediments. Indeed, in some systems, as much as 80% of the total reduced sulfur is present in the form of organic sulfur. It is important to petroleum geochemistry, carbon burial, and also the preservation of biomarkers used for molecularly-based paleoenvironmental reconstruction. Given that numerous modern and ancient depositional systems display organic sulfur enrichments, it is clear that the sequestration of organic sulfur could significantly affect the coupled biogeochemical cycles of sulfur, carbon, and oxygen on geological timescales. Nevertheless, a full measure of its importance to global biogeochemical cycles is only beginning to be recognized. The lack of study of organic sulfur is due mainly to the complexity of sedimentary S cycling, as well as to the extremely reactive nature of many intermediate inorganic S species that play a role in organic matter sulfurization, such as polysulfides, and the analytical difficulties inherent in the study of both these intermediates and organic sulfur compounds.
Recent analytical advances, particularly in the area of stable sulfur isotopic analysis, have provided avenues of investigation previously unavailable. My research has focused on the stable sulfur isotope composition (δ34S) of organic matter, both bulk and molecular forms (Werne et al., 2003, 2008). Because each sedimentary sulfur species has a sulfur isotopic composition that reflects the pathway of its formation, the δ34S of specific organic compounds measured in conjunction with that of coeval inorganic sulfur species can indicate the species of inorganic sulfur that is incorporated into organic matter, helping to elucidate the complex pathways of sulfur cycling in sedimentary and aqueous environments. As part of my research, I made the first measurements of the δ34S of specific organic compounds (Werne et al, 2008), which enhanced our understanding of the sources of sulfur and the processes surrounding organic sulfur formation. However, the methods I developed and utilized were time consuming and sample intensive. Recently, collaborators at Caltech have developed a method for analysis of compound-specific sulfur isotope analysis at natural abundance levels – by coupling a gas chromatograph to a multi-collector ICP-MS. This development opens the door for a much more extensive study of the organic sulfur, and its role in many different process within the coupled biogeochemical cycles of carbon and sulfur. Indeed, we have applied this technique to the same system in which I performed the initial molecular sulfur isotope work, and this improvement suggests multiple pathways by which organic matter reacts with sulfur in marine sediments (Raven et al., 2015).
My research in this area will continue to expand in the coming years, as I work to establish a baseline for the δ34S values of a broad suite of organic sulfur compounds, looking in particular for general patterns of fractionation among different types of molecules. For example, a current project funded by NSF focuses on the sulfurization of organic matter in two contrasting sulfidic lake systems. These systems are each considered analogs for the ocean on early Earth (during the Proterozoic, ~3 billion years ago), so this work could shed insight not only on this important biogeochemical process but also on biogeochemistry of early Earth systems. Once such a framework is established, much more refined questions will be able to be addressed with this new analytical capability. Another project, funded by the American Chemical Society – Petroleum Research Fund, is investigating the sulfur isotopic signature of sulfurization of carbohydrates in natural systems. This process has been implicated in the formation of unusually organic rich deposits in the geological past, such as the Jurassic aged Kimmeridge Clay formation, which is a well-known petroleum source rock.
Human environment interactions
A comparatively new and exciting area of research that I have become involved in since coming to Pitt is human-environment interactions. These studies range from recent anthropogenic impacts to studies of ancient cultures and their relationship with environmental and climatic conditions. For example, I am involved in an ongoing collaborative study called the “Hominin Sites and Paleolakes Drilling Project”, or HSPDP, funded by NSF. This study is an international scientific collaboration (more than 40 senior scientists from 8 countries) whose goal is to collect sediment drill cores for paleoclimate and paleoenviromental analysis in proximity to some of the world’s most important fossil hominin and artifact sites. To date, HSPDP has collected approximately 2,000 meters of lake sediments from key localities in Kenya and Ethiopia to vastly improve our understanding of the paleoenvironmental and paleoclimatic context of human evolution. My role in this study is to use molecular isotopic proxies to reconstruct climate and environmental changes such as temperature and rainfall that would have potentially influenced hominin development and migration. Using a combined data collection and modeling approach we aim to fundamentally transform the debate concerning how environmental dynamics at global, regional and local scales may have shaped hominin evolutionary history.
Another NSF-funded study that has just begun is in collaboration with Dr. Mark Abbott and Dr. Elizabeth Arkush (Dept. of Anthropology). This project, another unique outgrowth of the expertise gathered at Pitt, aims to use the abundance of specific biomarkers of mammal metabolism (fecal sterols) as indicators of human population and camelid pastoralism in early Andean societies. This approach has the potential to link climate and human populations, providing an unprecedented insight into pre-Columbian sociopolitical change and climate change.
The work broadly described here indicates some of the many ways in which my research has been and will continue to be applied to a wide variety of questions in (paleo)biogeochemistry, especially the response of ecosystems to both natural and anthropogenic climate and environmental change. These field and laboratory based, multiscale research activities are highly interdisciplinary and address questions central to climate broadly defined. Such results are important to understand the earth system, climate change, thresholds relevant to climate variation, and central to policies that address societal responses these changing systems on earth.
* denotes my advisee student/postdoc author
Berke*, M.A., T.C. Johnson, J.P. Werne, S. Schouten, J.S. Sinninghe Damsté (2012a) A mid-Holocene thermal maximum at the end of the African Humid Period. Earth & Planetary Science Letters v. 351-351, pp. 95-104.
Berke*, M.A., T.C. Johnson, J.P. Werne, K. Grice, S. Schouten, J.S. Sinninghe Damsté (2012b) Molecular records of climate variability and vegetation response since the Late Pleistocene in the Lake Victoria basin, East Africa. Quaternary Science Reviews v. 55, pp. 59-74.
Berke*, M.A., T.C. Johnson, J.P. Werne, D. Livingston, K. Grice, S. Schouten, J.S. Sinninghe Damsté (2014) Characterization of the last deglacial transition in tropical East Africa: Insights from Lake Albert, Africa. Palaeogeography, Palaeoclimatology, Palaeoecology. v. 409 pp. 1-8.
Blaga, Cornelia I., G.-J. Reichart, S. Schouten, A.F. Lotter, J.P. Werne, J.S. Sinninghe Damsté (2010) Branched glycerol dialkyl glycerol tetraethers in lake sediments: Can they be used as temperature and pH proxies? Organic Geochemistry, v. 41, pp. 1225-1234.
Brown, E.T., J.P. Werne, S. Lozano-Garcia, M. Caballero-Miranda, B. Ortega-Guerrero, E. Cabral Cano, B. L. Valero Garces, A. Schwalb (2013) Scientific drilling in the Basin of Mexico to evaluate climate history, hydrological resources, and seismic and volcanic hazards. Scientific Drilling v. 14, pp. 72-75.
Castañeda*, I., Werne, J.P., Johnson, T.C. (2007) Wet/arid phases in the southeast African tropics since the Last Glacial Maximum. Geology. v. 35, no. 9, pp. 823-826.
Castañeda*, I., Werne, J.P., Johnson, T. (2009a) Influence of climate change on algal community structure and primary productivity of Lake Malawi (East Africa) from the Last Glacial Maximum to present. Limnology & Oceanography, v. 54, no. 6/2, pp. 2431-2447.
Castañeda*, I., Werne, J.P., Johnson, T., and Filley, T. (2009b) Late Quaternary vegetation history of southeast Africa: the molecular isotopic record from Lake Malawi. Palaeogeography, Palaeoclimatology, Palaeoecology. v. 275, pp. 100-112.
Castañeda*, I., Werne, J.P., Johnson, T., Oberem*, L. (2011) Organic Geochemical Records from Lake Malawi (East Africa) of the last 700 years, part II: Biomarker Evidence for Recent Changes in Primary Productivity. Palaeogeography, Palaeoclimatology, Palaeoecology. v. 303, pp. 140-154.
Fawcett, P., J.P. Werne, R. Anderson, J. Heikoop, E. Brown, M. Berke*, S. Smith, F. Goff, L. Hurley, M. Cisneros-Dozal, S. Schouten, J. Sinninghe Damsté, Y. Huang, J. Toney, J. Fessenden, G. WoldeGabriel, V. Atudorei, J. Geissman, C. Allen (2011) Extended Megadroughts in the Southwestern United States during Pleistocene Interglacials. Nature. v. 470, pp. 518-521.
Kruger*, B.R., J.P. Werne, D.K. Branstrator, T.R. Hrabik, Y. Chikaraishi, N. Ohkouchi, E.C. Minor (2015) Organic matter transfer in Lake Superior’s food web: Insights from bulk and molecular stable isotope and radiocarbon analyses. Limnology & Oceanography. In Press
Powers*, L., Werne, J.P., Johnson, T.C., Hopmans, E.C., Sinninghe Damste, J.S.S., and Schouten, S. (2004) Crenarchaeotal membrane lipids in lake sediments: a new paleotemperature proxy for continental paleoclimate reconstruction? Geology, v. 32, no. 7, p. 613-616.
Powers*, L., T.C. Johnson, J.P. Werne, I. Castañeda*, E.C. Hopmans, J.S. Sinninghe Damsté, S. Schouten (2005) Large temperature variability in the southern African tropics since the Last Glacial Maximum. Geophysical Research Letters. 32, L08076, doi:10.1029/2004GL022014.
Powers*, L., J.P. Werne, A.J. Vanderwoude*, J.S. Sinninghe Damsté, E.C. Hopmans, S. Schouten (2010) Applicability and calibration of the TEX86 paleothermometer in lakes. Organic Geochemistry, v. 41, pp. 404-413.
Powers*, L., J.P. Werne, I. Castañeda*, T.C. Johnson, E.C. Hopmans, J.S. Sinninghe Damsté, S. Schouten (2011) Organic Geochemical Records of Environmental Variability in Lake Malawi During the Last 700 Years, Part I: The TEX86 Temperature Record. Palaeogeography, Palaeoclimatology, Palaeoecology. v. 303, pp. 133-139.
Raven*, M.R., A.L. Sessions, J.F. Adkins, T.W. Lyons, J.P. Werne (2015) Sulfur isotopic composition of individual organic compounds from Cariaco Basin sediments. Organic Geochemistry. v. 80 pp. 53-59.
Weijers, J.W.H., C. Blaga, J.P. Werne, and J.S. Sinninghe Damsté (2009) Microbial membrane lipids in lake sediments as a paleothermometer. PAGES News, v. 17, no. 3, pp. 102-104.
Weijers, J., B. Bernhardt*, F. Peterse, J.P. Werne, J.A.J. Dungait, S. Schouten, J.S. Sinninghe Damsté (2011) Absence of seasonal patterns in MBT-CBT indices in mid-latitude soils. Geochimica et Cosmochimica Acta. v. 75, pp. 3179-3190.
Werne, J.P., T.W. Lyons, D.J. Hollander, S. Schouten, E.C. Hopmans, and J.S. Sinninghe Damsté (2008) Investigating pathways of diagenetic organic matter sulfurization using compound-specific sulfur isotope analysis. Geochimica et Cosmochimica Acta, v. 72, pp. 3489-3502.
Werne, J.P., M. Baas, J.S. Sinninghe Damsté (2002) Molecular isotopic tracing of carbon flow and trophic relationships in a methane-supported microbial community. Limnology & Oceanography v. 46 no. 6, p. 1694-1701.
Werne, J.P., and J.S Sinninghe Damsté (2005) Mixed sources contribute to the molecular isotopic signature of methane-rich mud breccia sediments of Kazan mud volcano (Eastern Mediterranean). Organic geochemistry, v. 36, no. 1, pp. 13-27.
Werne, J.P., T. Zitter, R.R. Haese, G. Aloisi, I. Bouloubassi, S. Heijs, A. Fiala-Medioni, R.D. Pancost, J.S. Sinninghe Damste, G. de Lange, L.J. Forney, J.C. Gottschal, J.-P. Foucher, J. Mascle, J. Woodside, and the MEDINAUT and MEDINETH Shipboard Scientific Parties (2004) Life at cold seeps: A synthesis of ecological and biogeochemical data from Kazan mud volcano, eastern Mediterranean Sea. Chemical Geology, v. 205, no. 3-4, p. 367-390.
Werne, J.P., T.W. Lyons, D.J. Hollander, M.J. Formolo, J.S. Sinninghe Damsté (2003) Reduced sulfur in euxinic sediments of the Cariaco Basin: sulfur isotope constraints on organic sulfur formation. Chemical Geology v. 195, p. 159-179.
Woltering*, M., Werne, J.P., Johnson, T., S. Schouten, J.S. Sinninghe Damsté (2011) Late Pleistocene temperature history of southeast Africa: A TEX86 temperature record from Lake Malawi Palaeogeography, Palaeoclimatology, Palaeoecology. v. 303 p. 93-102.
Woltering*, M., J.P. Werne, J.L. Kish*, R. Hicks, J.S. Sinninghe Damsté, S. Schouten (2012) Vertical and temporal variability of Crenarchaeota in Lake Superior and the implications for the application of the TEX86 temperature proxy. Geochimica et Cosmochimica Acta v. 87, pp. 136-153.
Zigah*, P.K., E.C. Minor, J.P. Werne, S.L. McCallister (2011) Radiocarbon and stable-carbon isotopic insights into provenance and cycling of carbon in Lake Superior. Limnology & Oceanography. v. 56, pp. 867-886.
Zigah*, P.K., E.C. Minor, J.P. Werne (2012a) Radiocarbon and stable-isotope geochemistry of organic and inorganic carbon in Lake Superior. Global Biogeochemical Cycles. V. 26, GB1023, doi:10.1029/2011GB004132
Zigah*, P.K., E.C. Minor, J.P. Werne, S. Leigh McCallister (2012b) An isotopic (Δ14C, δ13C, and δ15N) investigation of zooplankton allochthony vs. autochthony in Lake Superior and across a size-gradient of aquatic systems. Biogeosciences v. 9, pp. 3663-3678.