Honors & Awards
Explorer, National Geographic (2017)
Ocean Sciences Fellowship, Alfred P. Sloan Foundation (2017)
Pre-tenure award, GSA Geobiology and Geomicrobiology Division (2016)
NAI Postdoctoral Fellowship, NASA Astrobiology Institute (2012-2014)
Geobiology Fellowship, Agouron Institute (2010-2012)
Boards, Advisory Committees, Professional Organizations
Editorial Board, Palaios (2017 - Present)
Editorial Advisory Board, Geobiology (2016 - Present)
Current Research and Scholarly Interests
The research interests in the Sperling Lab are Earth history and the evolution of life, and the interactions between the biosphere and the geosphere. As such this research can generally be considered paleontology, insofar as paleontology encompasses all aspects of the history of life.
Consequently, we define our research agenda by the questions we are interested in, rather than the tools used. This research incorporates multiple lines of evidence, and multiple tools, to investigate questions in the history of life. These lines of evidence include fossil data, molecular phylogenetics, sedimentary geochemistry, and developmental and ecological data from modern organisms. Ultimately, the goal is to link environmental change with organismal and ecological response through the lens of physiology.
Our field research takes place all over the world--current areas include:
-NW Canada (Yukon and Northwest Territories): Research has been conducted on the early Neoproterozoic Fifteenmile Group, Cryogenian and Ediacaran Windermere Supergroup, and on the Ordovician-Devonian Road River Group in the southern Richardson Mountains
-Southern Canadian Cordillera: Work here has focused on the early Cambrian Mural Formation and its soft-bodied fauna.
-England and Wales: Cambrian-Silurian successions in the Welsh Basin
-Namibia: Ediacaran Nama Group
-Upwelling zones: We study the oxygen minimum zone offshore California as an analogue for ancient low-oxygen oceans.
- Introduction to Geology
EARTHSYS 11, GEOLSCI 1 (Spr)
- Topics in Organismal Paleobiology
GEOLSCI 206 (Win)
Independent Studies (7)
- Advanced Projects
GEOLSCI 399 (Win, Spr)
- Directed Reading with Geological Sciences Faculty
GEOLSCI 292 (Win)
- Graduate Research
GEOLSCI 400 (Aut, Win, Spr, Sum)
- Honors Program
GEOLSCI 199 (Win, Spr)
- Honors Program in Earth Systems
EARTHSYS 199 (Win, Spr)
- Practical Experience in the Geosciences
GEOLSCI 385 (Sum)
- Teaching in Geological Sciences
GEOLSCI 398 (Spr)
- Advanced Projects
Prior Year Courses
- Departmental Seminar in Geological Sciences
GEOLSCI 290 (Aut, Win, Spr)
- Fundamentals of Geobiology
ESS 205, GEOLSCI 205 (Aut)
- Geology of Oman Field Trip
GEOLSCI 293A (Aut)
- Introduction to Geology
GEOLSCI 1 (Spr)
- Introduction to Geology
EARTHSYS 11, GS 1 (Win)
- Sedimentary Geochemistry Field Trip
GS 135A (Spr)
- Sedimentary Geochemistry and Analysis
GS 135, GS 235 (Spr)
- Departmental Seminar in Geological Sciences
Doctoral Dissertation Reader (AC)
Postdoctoral Faculty Sponsor
Murray Duncan, Daniel Mills
Doctoral Dissertation Advisor (AC)
Tom Boag, Malcolm Hodgskiss, Samantha Ritzer, Richard Stockey
Master's Program Advisor
Doctoral Dissertation Co-Advisor (AC)
Ediacaran reorganization of the marine phosphorus cycle.
Proceedings of the National Academy of Sciences of the United States of America
The Ediacaran Period (635 to 541 Ma) marks the global transition to a more productive biosphere, evidenced by increased availability of food and oxidants, the appearance of macroscopic animals, significant populations of eukaryotic phytoplankton, and the onset of massive phosphorite deposition. We propose this entire suite of changes results from an increase in the size of the deep-water marine phosphorus reservoir, associated with rising sulfate concentrations and increased remineralization of organic P by sulfate-reducing bacteria. Simple mass balance calculations, constrained by modern anoxic basins, suggest that deep-water phosphate concentrations may have increased by an order of magnitude without any increase in the rate of P input from the continents. Strikingly, despite a major shift in phosphorite deposition, a new compilation of the phosphorus content of Neoproterozoic and early Paleozoic shows little secular change in median values, supporting the view that changes in remineralization and not erosional P fluxes were the principal drivers of observed shifts in phosphorite accumulation. The trigger for these changes may have been transient Neoproterozoic weathering events whose biogeochemical consequences were sustained by a set of positive feedbacks, mediated by the oxygen and sulfur cycles, that led to permanent state change in biogeochemical cycling, primary production, and biological diversity by the end of the Ediacaran Period.
View details for DOI 10.1073/pnas.1916738117
View details for PubMedID 32424088
Persistent global marine euxinia in the early Silurian.
2020; 11 (1): 1804
The second pulse of the Late Ordovician mass extinction occurred around the Hirnantian-Rhuddanian boundary (~444Ma) and has been correlated with expanded marine anoxia lasting into the earliest Silurian. Characterization of the Hirnantian ocean anoxic event has focused on the onset of anoxia, with global reconstructions based on carbonate delta238U modeling. However, there have been limited attempts to quantify uncertainty in metal isotope mass balance approaches. Here, we probabilistically evaluate coupled metal isotopes and sedimentary archives to increase constraint. We present iron speciation, metal concentration, delta98Mo and delta238U measurements of Rhuddanian black shales from the Murzuq Basin, Libya. We evaluate these data (and published carbonate delta238U data) with a coupled stochastic mass balance model. Combined statistical analysis of metal isotopes and sedimentary sinks provides uncertainty-bounded constraints on the intensity of Hirnantian-Rhuddanian euxinia. This work extends the duration of anoxia to >3 Myrs - notably longer than well-studied Mesozoic ocean anoxic events.
View details for DOI 10.1038/s41467-020-15400-y
View details for PubMedID 32286253
On the co-evolution of surface oxygen levels and animals.
Few topics in geobiology have been as extensively debated as the role of Earth's oxygenation in controlling when and why animals emerged and diversified. All currently described animals require oxygen for at least a portion of their life cycle. Therefore, the transition to an oxygenated planet was a prerequisite for the emergence of animals. Yet, our understanding of Earth's oxygenation and the environmental requirements of animal habitability and ecological success is currently limited; estimates for the timing of the appearance of environments sufficiently oxygenated to support ecologically stable populations of animals span a wide range, from billions of years to only a few million years before animals appear in the fossil record. In this light, the extent to which oxygen played an important role in controlling when animals appeared remains a topic of debate. When animals originated and when they diversified are separate questions, meaning either one or both of these phenomena could have been decoupled from oxygenation. Here, we present views from across this interpretive spectrum-in a point-counterpoint format-regarding crucial aspects of the potential links between animals and surface oxygen levels. We highlight areas where the standard discourse on this topic requires a change of course and note that several traditional arguments in this "life versus environment" debate are poorly founded. We also identify a clear need for basic research across a range of fields to disentangle the relationships between oxygen availability and emergence and diversification of animal life.
View details for DOI 10.1111/gbi.12382
View details for PubMedID 32175670
- Sources of C30 steroid biomarkers in Neoproterozoic-Cambrian rocks and oils. Nature ecology & evolution 2019
- New insights on the Orosirian carbon cycle, early Cyanobacteria, and the assembly of Laurentia from the Paleoproterozoic Belcher Group EARTH AND PLANETARY SCIENCE LETTERS 2019; 520: 141–52
- Statistical inference and reproducibility in geobiology GEOBIOLOGY 2019; 17 (3): 261–71
Oxygenated Mesoproterozoic lake revealed through magnetic mineralogy.
Proceedings of the National Academy of Sciences of the United States of America
Terrestrial environments have been suggested as an oxic haven for eukaryotic life and diversification during portions of the Proterozoic Eon when the ocean was dominantly anoxic. However, iron speciation and Fe/Al data from the ca. 1.1-billion-year-old Nonesuch Formation, deposited in a large lake and bearing a diverse assemblage of early eukaryotes, are interpreted to indicate persistently anoxic conditions. To shed light on these distinct hypotheses, we analyzed two drill cores spanning the transgression into the lake and its subsequent shallowing. While the proportion of highly reactive to total iron (FeHR/FeT) is consistent through the sediments and typically in the range taken to be equivocal between anoxic and oxic conditions, magnetic experiments and petrographic data reveal that iron exists in three distinct mineral assemblages resulting from an oxycline. In the deepest waters, reductive dissolution of iron oxides records an anoxic environment. However, the remainder of the sedimentary succession has iron oxide assemblages indicative of an oxygenated environment. At intermediate water depths, a mixed-phase facies with hematite and magnetite indicates low oxygen conditions. In the shallowest waters of the lake, nearly every iron oxide has been oxidized to its most oxidized form, hematite. Combining magnetics and textural analyses results in a more nuanced understanding of ambiguous geochemical signals and indicates that for much of its temporal duration, and throughout much of its water column, there was oxygen in the waters of Paleolake Nonesuch.
View details for PubMedID 30509974
- Oxygen, temperature and the deep-marine stenothermal cradle of Ediacaran evolution PROCEEDINGS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES 2018; 285 (1893)
Demosponge steroid biomarker 26-methylstigmastane provides evidence for Neoproterozoic animals.
Nature ecology & evolution
2018; 2 (11): 1709–14
Sterane biomarkers preserved in ancient sedimentary rocks hold promise for tracking the diversification and ecological expansion of eukaryotes. The earliest proposed animal biomarkers from demosponges (Demospongiae) are recorded in a sequence around 100Myr long of Neoproterozoic-Cambrian marine sedimentary strata from the Huqf Supergroup, South Oman Salt Basin. This C30 sterane biomarker, informally known as 24-isopropylcholestane (24-ipc), possesses the same carbon skeleton as sterols found in some modern-day demosponges. However, this evidence is controversial because 24-ipc is not exclusive to demosponges since 24-ipc sterols are found in trace amounts in some pelagophyte algae. Here, we report a new fossil sterane biomarker that co-occurs with 24-ipc in a suite of late Neoproterozoic-Cambrian sedimentary rocks and oils, which possesses a rare hydrocarbon skeleton that is uniquely found within extant demosponge taxa. This sterane is informally designated as 26-methylstigmastane (26-mes), reflecting the very unusual methylation at the terminus of the steroid side chain. It is the first animal-specific sterane marker detected in the geological record that can be unambiguously linked to precursor sterols only reported from extant demosponges. These new findings strongly suggest that demosponges, and hence multicellular animals, were prominent in some late Neoproterozoic marine environments at least extending back to the Cryogenian period.
View details for PubMedID 30323207
- Demosponge steroid biomarker 26-methylstigmastane provides evidence for Neoproterozoic animals NATURE ECOLOGY & EVOLUTION 2018; 2 (11): 1709–14
The Temporal and Environmental Context of Early Animal Evolution: Considering All the Ingredients of an "Explosion".
Integrative and comparative biology
2018; 58 (4): 605–22
Animals originated and evolved during a unique time in Earth history-the Neoproterozoic Era. This paper aims to discuss (1) when landmark events in early animal evolution occurred, and (2) the environmental context of these evolutionary milestones, and how such factors may have affected ecosystems and body plans. With respect to timing, molecular clock studies-utilizing a diversity of methodologies-agree that animal multicellularity had arisen by 800 million years ago (Ma) (Tonian period), the bilaterian body plan by 650 Ma (Cryogenian), and divergences between sister phyla occurred 560-540 Ma (late Ediacaran). Most purported Tonian and Cryogenian animal body fossils are unlikely to be correctly identified, but independent support for the presence of pre-Ediacaran animals is recorded by organic geochemical biomarkers produced by demosponges. This view of animal origins contrasts with data from the fossil record, and the taphonomic question of why animals were not preserved (if present) remains unresolved. Neoproterozoic environments demanding small, thin, body plans, and lower abundance/rarity in populations may have played a role. Considering environmental conditions, geochemical data suggest that animals evolved in a relatively low-oxygen ocean. Here, we present new analyses of sedimentary total organic carbon contents in shales suggesting that the Neoproterozoic ocean may also have had lower primary productivity-or at least lower quantities of organic carbon reaching the seafloor-compared with the Phanerozoic. Indeed, recent modeling efforts suggest that low primary productivity is an expected corollary of a low-O2 world. Combined with an inability to inhabit productive regions in a low-O2 ocean, earliest animal communities would likely have been more food limited than generally appreciated, impacting both ecosystem structure and organismal behavior. In light of this, we propose the "fire triangle" metaphor for environmental influences on early animal evolution. Moving toward consideration of all environmental aspects of the Cambrian radiation (fuel, heat, and oxidant) will ultimately lead to a more holistic view of the event.
View details for PubMedID 30295813
Oxygen, temperature and the deep-marine stenothermal cradle of Ediacaran evolution.
Proceedings. Biological sciences
2018; 285 (1893): 20181724
Ediacaran fossils document the early evolution of complex megascopic life, contemporaneous with geochemical evidence for widespread marine anoxia. These data suggest early animals experienced frequent hypoxia. Research has thus focused on the concentration of molecular oxygen (O2) required by early animals, while also considering the impacts of climate. One model, the Cold Cradle hypothesis, proposed the Ediacaran biota originated in cold, shallow-water environments owing to increased O2 solubility. First, we demonstrate using principles of gas exchange that temperature does have a critical role in governing the bioavailability of O2-but in cooler water the supply of O2 is actually lower. Second, the fossil record suggests the Ediacara biota initially occur approximately 571 Ma in deep-water facies, before appearing in shelf environments approximately 555 Ma. We propose an ecophysiological underpinning for this pattern. By combining oceanographic data with new respirometry experiments we show that in the shallow mixed layer where seasonal temperatures fluctuate widely, thermal and partial pressure ( pO2) effects are highly synergistic. The result is that temperature change away from species-specific optima impairs tolerance to low pO2. We hypothesize that deep and particularly stenothermal (narrow temperature range) environments in the Ediacaran ocean were a physiological refuge from the synergistic effects of temperature and low pO2.
View details for PubMedID 30963899
View details for PubMedCentralID PMC6304043
On the edge of exceptional preservation: insights into the role of redox state in Burgess Shale-type taphonomic windows from the Mural Formation, Alberta, Canada.
Emerging topics in life sciences
2018; 2 (2): 311–23
Animals originated in the Neoproterozoic and 'exploded' into the fossil record in the Cambrian. The Cambrian also represents a high point in the animal fossil record for the preservation of soft tissues that are normally degraded. Specifically, fossils from Burgess Shale-type (BST) preservational windows give paleontologists an unparalleled view into early animal evolution. Why this time interval hosts such exceptional preservation, and why this preservational window declines in the early Paleozoic, have been long-standing questions. Anoxic conditions have been hypothesized to play a role in BST preservation, but recent geochemical investigations of these deposits have reached contradictory results with respect to the redox state of overlying bottom waters. Here, we report a multi-proxy geochemical study of the Lower Cambrian Mural Formation, Alberta, Canada. At the type section, the Mural Formation preserves rare recalcitrant organic tissues in shales that were deposited near storm wave base (a Tier 3 deposit; the worst level of soft-tissue preservation). The geochemical signature of this section shows little to no evidence of anoxic conditions, in contrast with published multi-proxy studies of more celebrated Tier 1 and 2 deposits. These data help confirm that 'decay-limited' BST biotas were deposited in more oxygenated conditions, and support a role for anoxic conditions in BST preservation. Finally, we discuss the role of iron reduction in BST preservation, including the formation of iron-rich clays and inducement of sealing seafloor carbonate cements. As oceans and sediment columns became more oxygenated and more sulfidic through the early Paleozoic, these geochemical changes may have helped close the BST taphonomic window.
View details for DOI 10.1042/ETLS20170163
View details for PubMedID 32412614
Temperature-dependent hypoxia explains biogeography and severity of end-Permian marine mass extinction.
Science (New York, N.Y.)
2018; 362 (6419)
Rapid climate change at the end of the Permian Period (~252 million years ago) is the hypothesized trigger for the largest mass extinction in Earth's history. We present model simulations of the Permian/Triassic climate transition that reproduce the ocean warming and oxygen (O2) loss indicated by the geologic record. The effect of these changes on animal survival is evaluated using the Metabolic Index (Phi), a measure of scope for aerobic activity governed by organismal traits sampled in diverse modern species. Modeled loss of aerobic habitat predicts lower extinction intensity in the tropics, a pattern confirmed with a spatially explicit analysis of the marine fossil record. The combined physiological stresses of ocean warming and O2 loss can account for more than half the magnitude of the "Great Dying."
View details for PubMedID 30523082
- Oxygen, facies, and secular controls on the appearance of Cryogenian and Ediacaran body and trace fossils in the Mackenzie Mountains of northwestern Canada GEOLOGICAL SOCIETY OF AMERICA BULLETIN 2016; 128 (3-4): 558-575
Biotic replacement and mass extinction of the Ediacara biota.
Proceedings. Biological sciences / The Royal Society
2015; 282 (1814)
The latest Neoproterozoic extinction of the Ediacara biota has been variously attributed to catastrophic removal by perturbations to global geochemical cycles, 'biotic replacement' by Cambrian-type ecosystem engineers, and a taphonomic artefact. We perform the first critical test of the 'biotic replacement' hypothesis using combined palaeoecological and geochemical data collected from the youngest Ediacaran strata in southern Namibia. We find that, even after accounting for a variety of potential sampling and taphonomic biases, the Ediacaran assemblage preserved at Farm Swartpunt has significantly lower genus richness than older assemblages. Geochemical and sedimentological analyses confirm an oxygenated and non-restricted palaeoenvironment for fossil-bearing sediments, thus suggesting that oxygen stress and/or hypersalinity are unlikely to be responsible for the low diversity of communities preserved at Swartpunt. These combined analyses suggest depauperate communities characterized the latest Ediacaran and provide the first quantitative support for the biotic replacement model for the end of the Ediacara biota. Although more sites (especially those recording different palaeoenvironments) are undoubtedly needed, this study provides the first quantitative palaeoecological evidence to suggest that evolutionary innovation, ecosystem engineering and biological interactions may have ultimately caused the first mass extinction of complex life.
View details for DOI 10.1098/rspb.2015.1003
View details for PubMedID 26336166
View details for PubMedCentralID PMC4571692
- Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation NATURE 2015; 523 (7561): 451-454
- The Ecological Physiology of Earth's Second Oxygen Revolution ANNUAL REVIEW OF ECOLOGY, EVOLUTION, AND SYSTEMATICS, VOL 46 2015; 46: 215-235