Cole T. Edwards

Persistent oceanic anoxia and elevated extinction rates separate the Cambrian and Ordovician radiations

Recurrent mass extinction events (at “biomere”—a biostratigraphic unit—boundaries) characterize the middle Cambrian to Early Ordovician (Tremadocian) time interval that is between the major Cambrian and Ordovician radiations of animal life. A role for anoxia in maintaining elevated extinction rates in the late Cambrian has been proposed based on coincidence of an extinction with positive excursions in δ13Ccarb and δ34SCAS (CAS—carbonate-associated sulfate). Here we examine an Early Ordovician extinction event at the base of the North American Stairsian Stage (upper Tremadocian), and demonstrate concurrent onset of positive excursions in δ13C and δ34S inferred to reflect enhanced organic matter burial under anoxic waters. Sea-level rise may have brought anoxic waters onto the shelf to initiate extinctions. The evidence for δ13C excursions and elevated extinction rates appears to wane in the Tremadocian, consistent with progressive oxygenation of the oceans reaching a threshold that helped facilitate initial stages of the Great Ordovician Biodiversification Event.

Calibration of a conodont apatite-based Ordovician 87Sr/86Sr curve to biostratigraphy and geochronology: Implications for stratigraphic resolution

The Ordovician 87 Sr/ 86 Sr isotope seawater curve is well established and shows a decreasing trend until the mid-Katian. However, uncertainties in calibration of this curve to biostratigraphy and geochronology have made it diffi cult to determine how the rates of 87 Sr/ 86 Sr decrease may have varied, which has implications for both the stratigraphic resolution possible using Sr isotope stratigraphy and efforts to model the effects of Ordovician geologic events. We measured 87 Sr/ 86 Sr in conodont apatite in North American Ordovician sections that are well studied for conodont biostratigraphy, primarily in Nevada, Oklahoma, the Appalachian region, and Ohio Valley. Our results indicate that conodont apatite may provide an accurate medium for Sr isotope stratigraphy and strengthen previous reports that point toward a signifi cant increase in the rate of fall in seawater 87 Sr/ 86 Sr during the Middle Ordovician Darriwilian Stage. Our 87 Sr/ 86 Sr results suggest that Sr isotope stratigraphy will be most useful as a high-resolution tool for global correlation in the mid-Darriwilian to mid-Sandbian, when the maximum rate of fall in Sr/

Strontium isotope (87Sr/86Sr) stratigraphy of Ordovician bulk carbonate: Implications for preservation of primary seawater values

The present study on bulk carbonate 87 Sr/ 86 Sr stratigraphy represents a companion work to earlier research that presented a conodont apatite-based Ordovician seawater 87 Sr/ 86 Sr curve for the Tremadocian–Katian Stages (485–445 Ma). Here, we directly compare the curve based on conodont apatite (including some new data not published in earlier work) with a new curve based on 87 Sr/ 86 Sr results from bulk carbonate from the Tremadocian–Sandbian Stages. We sampled eight Lower to Upper Ordovician carbonate successions in North America to assess the reliability of bulk carbonate to preserve seawater 87 Sr/ 86 Sr and its utility for 87 Sr/ 86 Sr chemostratigraphy. A high-resolution 87 Sr/ 86 Sr curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater 87 Sr/ 86 Sr ( 87 Sr/ 86 Sr seawater ). In total, 230 bulk carbonate samples that are paired to conodont samples were measured for 87 Sr/ 86 Sr in order to determine the conditions under which 87 Sr/ 86 Sr seawater is preserved in bulk carbonate. Results indicate that well-preserved bulk carbonate can faithfully record the 87 Sr/ 86 Sr seawater trend, but that its 87 Sr/ 86 Sr values are commonly more variable than those of conodont apatite. On average, bulk carbonate samples of the same age vary by 10–20 × 10 −5 , compared to 5–10 × 10 −5 for conodont apatite. The amount of isotopic alteration of bulk carbonate from seawater 87 Sr/ 86 Sr (Δ 87 Sr/ 86 Sr) was determined by taking the difference between 87 Sr/ 86 Sr values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont 87 Sr/ 86 Sr values. Cross plots comparing Δ 87 Sr/ 86 Sr values to bulk carbonate Sr concentration ([Sr]) and conodont color alteration indices (CAI; an estimate of the thermal history of a rock body) indicate that bulk carbonate is most likely to preserve 87 Sr/ 86 Sr seawater (minimally altered) when either: (1) bulk carbonate [Sr] is greater than 300 ppm, or (2) carbonate rocks experienced minimal thermal alteration, with burial temperatures less than ~150 °C. Carbonates with intermediate [Sr] (e.g., between 130 and 300 ppm) can also yield 87 Sr/ 86 Sr seawater values, but results are less predictable, and local diagenetic conditions may play a greater role. Modeling results support the argument that seawater 87 Sr/ 86 Sr can be preserved in bulk carbonates with low [Sr] if pore water:rock ratios are low ( 87 Sr/ 86 Sr is similar to the seawater 87 Sr/ 86 Sr value preserved in limestone. Bulk carbonate samples that meet these criteria can be useful for high-resolution measurements of 87 Sr/ 86 Sr seawater , with a sample variation on par with fossil materials ( −5 ), particularly for successions where well-preserved fossil material (i.e., conodonts or brachiopods) is not available, such as Precambrian strata, sequences recording mass extinction events, or otherwise fossil-barren facies. These criteria and model predictions based on bulk carbonate [Sr] must be considered in the context of whether a limestone accumulated under calcite seas (e.g., Ordovician), with relatively high seawater Sr/Ca, or aragonite seas, in which case the diagenetic transformation of aragonite to calcite may result in incorporation of non-seawater Sr.

Oxygenation as a driver of the Great Ordovician Biodiversification Event

The largest radiation of Phanerozoic marine animal life quadrupled genus-level diversity towards the end of the Ordovician Period about 450 million years ago. A leading hypothesis for this Great Ordovician Biodiversification Event is that cooling of the Ordovician climate lowered sea surface temperatures into the thermal tolerance window of many animal groups, such as corals. A complementary role for oxygenation of subsurface environments has been inferred based on the increasing abundance of skeletal carbonate, but direct constraints on atmospheric O2 levels remain elusive. Here, we use high-resolution paired bulk carbonate and organic carbon isotope records to determine the changes in isotopic fractionation between these phases throughout the Ordovician radiation. These results can be used to reconstruct atmospheric O2 levels based on the O2-dependent fractionation of carbon isotopes by photosynthesis. We find a strong temporal link between the Great Ordovician Biodiversification Event and rising O2 concentrations, a pattern that is corroborated by O2 models that use traditional carbon–sulfur mass balance. We conclude that that oxygen levels probably played an important role in regulating early Palaeozoic biodiversity levels, even after the Cambrian Explosion.An increase in biodiversity 450 million years ago coincided with a rise in atmospheric oxygen concentrations, suggests a geochemical analysis. Oxygen availability may have thus helped spur the radiation alongside climatic cooling.

Late inception of a resiliently oxygenated upper ocean

The rise of oxygen To understand the evolution of the biosphere, we need to know how much oxygen was present in Earths atmosphere during most of the past 2.5 billion years. However, there are few proxies sensitive enough to quantify O2 at the low levels present until slightly less than 1 billion years ago. Lu et al. measured iodine/calcium ratios in marine carbonates, which are a proxy for dissolved oxygen concentrations in the upper ocean. They found that a major, but temporary, rise in atmospheric O2 occurred at around 400 million years ago and that O2 levels underwent a step change to near-modern values around 200 million years ago. Science, this issue p. 174 The I/Ca ratio in marine carbonates tracks atmospheric oxygen levels for the past 2.5 billion years. Rising oceanic and atmospheric oxygen levels through time have been crucial to enhanced habitability of surface Earth environments. Few redox proxies can track secular variations in dissolved oxygen concentrations around threshold levels for metazoan survival in the upper ocean. We present an extensive compilation of iodine-to-calcium ratios (I/Ca) in marine carbonates. Our record supports a major rise in the partial pressure of oxygen in the atmosphere at ~400 million years (Ma) ago and reveals a step change in the oxygenation of the upper ocean to relatively sustainable near-modern conditions at ~200 Ma ago. An Earth system model demonstrates that a shift in organic matter remineralization to greater depths, which may have been due to increasing size and biomineralization of eukaryotic plankton, likely drove the I/Ca signals at ~200 Ma ago.