Matthew R. Saltzman

Geochemical evidence for widespread euxinia in the Later Cambrian ocean

Widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long-term inhibitor of evolutionary radiation on early Earth. In recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later Cambrian oceans, possibly influencing evolutionary events during this time. Physical evidence of widespread anoxia in Cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored. Here we present sulphur isotope records from six globally distributed stratigraphic sections of later Cambrian marine rocks (about 499 million years old). We find a positive sulphur isotope excursion in phase with the Steptoean Positive Carbon Isotope Excursion (SPICE), a large and rapid excursion in the marine carbon isotope record, which is thought to be indicative of a global carbon cycle perturbation. Numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large-scale anoxic and sulphidic (euxinic) conditions. Independently, molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia. These results identify the SPICE interval as the best characterized ocean anoxic event in the pre-Mesozoic ocean and an extreme example of oxygen deficiency in the later Cambrian ocean. Thus, a redox structure similar to those in Proterozoic oceans may have persisted or returned in the oceans of the early Phanerozoic eon. Indeed, the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in Cambrian oceans.

Long-lived glaciation in the Late Ordovician? Isotopic and sequence-stratigraphic evidence from western Laurentia

The timing and causes of the transition to an icehouse climate in the Late Ordovician are controversial. Results of an integrated δ13C and sequence stratigraphic analysis in Nevada show that in the Late Ordovician Chatfieldian Stage (mid-Caradoc) a positive δ13C excursion in the upper part of the Copenhagen Formation was closely followed by a regressive event evidenced within the prominent Eureka Quartzite. The Chatfieldian δ13C excursion is known globally and interpreted to record enhanced organic carbon burial, which lowered atmospheric p CO2 to levels near the threshold for ice buildup in the Ordovician greenhouse climate. The subsequent regressive event in central Nevada, previously interpreted as part of a regional tectonic adjustment, is here attributed in part to sea-level drawdown from the initiation of continental glaciation on Gondwana. This drop in sea level—which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of p CO2 by suppressing shelf-carbonate production—signals the transition to a Late Ordovician icehouse climate ∼10 m.y. before the widespread Hirnantian glacial maximum at the end of the Ordovician.

Phosphorus, nitrogen, and the redox evolution of the Paleozoic oceans

A new high-resolution Paleozoic d 13 Ccarb curve from the Great Basin shows an amount of variation that appears transitional between the highly unsettled Neoproterozoic and the increasingly stable Mesozoic to Cenozoic periods. Large positive excursions were common during cool periods (e.g., Late Ordovician‐Silurian and Late Devonian‐Early Mississippian), but rare during greenhouse climates. Some periods of stability in d 13 Ccarb lasted for .10 7 yr and are interpreted to reflect negative feedbacks on productivity in a nitrogen

A global carbon isotope excursion (SPICE) during the Late Cambrian: relation to trilobite extinctions, organic-matter burial and sea level

Abstract The Steptoean positive carbon isotope excursion (SPICE) marks a global oceanographic event that confirms intercontinental correlations between different biogeographic realms based on agnostids and other blue-water trilobites. The SPICE excursion is documented from sections in Laurentia, Kazakhstan, China, and Australia where it begins with the mass extinction at the base of the Pterocephaliid Biomere (Steptoean Stage) in Laurentia and at coeval extinction horizons in Gondwana and periGondwana terranes. The peak of SPICE (+5‰) coincided with a time of maximum regression in Laurentia. SPICE is similar in this regard to excursions that coincide with glacio-eustatic falls, such as in the Late Ordovician. A plausible scenario involves the transformation of ocean circulation between two states, which led to enhanced coastal upwelling and benthic extinctions. The lack of evidence for glaciation indicates that the coeval sea level fall (Sauk II–Sauk III event) resulted from tectonic or hydrologic changes that remain poorly understood at this time.

CARBON ISOTOPE STRATIGRAPHY OF UPPER CAMBRIAN (STEPTOEAN STAGE) SEQUENCES OF THE EASTERN GREAT BASIN : RECORD OF A GLOBAL OCEANOGRAPHIC EVENT

A large, global positive excursion in δ 13 C (from ∼0.5 to 4.5‰) during the Late Cambrian Pterocephaliid biomere/Steptoean Stage (Aphelaspis-Elvinia zones) is documented at high stratigraphic resolution in three sections in the eastern Great Basin. The excursion, which we refer to as the Steptoean Positive Carbon Isotope Excursion, or SPICE excursion, began coincident with a world-wide extinction event. The δ 13 C data from the Great Basin reflect global seawater values in a wide range of lithologies, including oolitic grainstones, wackestones, thrombolitic boundstones, and flat-pebble conglomerates. We use a section at Shingle Pass in the southern Egan Range to divide the Pterocephaliid biomere into eight isotope steps that represent equal divisions of the δ 13 C rise and fall (±4‰). This provides a basis for recognition of a revised chronostratigraphic framework for the Pterocephaliid biomere/Steptoean Stage. Strata deposited during the beginning of the SPICE excursion record a major change in the pattern of sedimentation in the eastern Great Basin. This is reflected in a siliciclastic-carbonate transition at Shingle Pass, Nevada, and a carbonate-siliciclastic transition at the House Range and Lawson Cove sections in Utah. A regional siliciclastic influx recognized throughout the Great Basin occurs near the peak of the SPICE excursion. Carbon isotope analyses from cratonal sections in Wyoming provide independent evidence that a major sedimentary hiatus took place on the craton during the time of the SPICE excursion. The correlated changes in δ 13 C, relative sea level, and the marine biota during the SPICE excursion provide remarkably detailed records of a major paleoceanographic event. We speculate that changes in sea level, climate, or tectonics may have triggered the SPICE excursion and coeval extinction event. Subsequent burial of organic carbon caused the increase in δ 13 C and may have led to an interval of global cooling. The results of this study lend confidence to carbon-isotopic studies of pre-Mesozoic rocks.

Chapter 11 – Carbon Isotope Stratigraphy

Abstract: Variations in the 13 C/ 12 C value of total dissolved inorganic carbon (DIC) in the world’s oceans through time have been documented through stratigraphic study of marine carbonate rocks (δ 13 C carb ). This variation has been used to date and correlate sediments. The stratigraphic record of carbon isotopes is complex because the main process fractionating 12 C from 13 C is photosynthesis, with organic matter depleted in the heavy isotope ( 13 C). The carbon isotope record (on the geological time scales considered here) is to a large extent defined by changes in the partitioning of carbon between organic carbon and carbonate, and therefore linked directly to the biosphere and the global carbon cycle. This chapter summarizes δ 13 C carb variations through geologic time compiled from multiple literature sources. Materials analyzed for curve-construction differ between authors and between geological time periods, and one should carefully consider whether skeletal carbonate secreted by specific organisms or bulk carbonate has been used in evaluating or comparing carbon isotope stratigraphic records. Mid-Jurassic through Cenozoic curves have been mainly derived from pelagic carbonates, and exhibit low amplitude δ 13 C carb variability (from −1 to +4‰) relative to curves for the earlier part of the record (from −3 to +8 ‰ for the Phanerozoic, from −15 to +15‰ for the Proterozoic and Archean). The Mid-Jurassic and older curves are dominantly based on data from platform carbonates, which show greater variability and more spatial heterogeneity. The different character of carbon isotope curves derived from older platform carbonates as compared to younger pelagic records may reflect primary and/or diagenetic processes, difference in paleoenvironments, difference in calcifying organisms, or inherent changes in the global carbon cycle with geologic time and biotic evolution (e.g., changes in reservoir size).

Global Standard Stratotype-section and Point (GSSP) of the Furongian Series and Paibian Stage (Cambrian)

The Global Standard Stratotype-section and Point (GSSP) of the Furongian Series (uppermost series of the Cambrian System) and the Paibian Stage (lowermost stage of the Furongian Series), has been recently defined and ratified by the International Union of Geological Sciences (IUGS). The boundary stratotype is 369 metres above the base of the Huaqiao Formation in the Paibi section, northwestern Hunan Province, China. This point coincides with the first appearance of the cosmopolitan agnostoid trilobite Glyptagnostus reticulatus, and occurs near the base of a large positive carbon isotopic excursion (SPICE excursion).

Late Paleozoic ice age: Oceanic gateway or pCO2?

The cause of the late Paleozoic (ca. 355-255 Ma) ice age remains uncertain. A lowering of atmospheric carbon dioxide levels near the beginning of this time period occurred in response to the rise of land plants and likely cooled Earth, but the rapid growth of extensive Gondwanan ice sheets was delayed for tens of millions of years, until the Late Mississippian. The δ 1 3 C values from a thick succession at Arrow Canyon, Nevada, indicate a divergence between North America and Europe (∼2‰) across the Mississippian-Pennsylvanian transition, and support a scenario in which the closure of a subequatorial oceanic gateway during the assembly of Pangea altered the oceanic distribution of nutrients ( 1 2 C) and led to enhanced poleward transport of heat and moisture. This change marks the transition from a cool, moisture-starved Gondwana to the icehouse world of the Pennsylvanian and Early Permian.

The Late Cambrian SPICE (δ13C) event and the Sauk II-Sauk III regression: new evidence from Laurentian basins in Utah, Iowa, and Newfoundland

Carbon isotope data from Upper Cambrian sections in three Laurentian basins in northern Utah, central Iowa, and western Newfoundland record a large positive d 13 C excursion (SPICE event) of up to 1 5‰. Peak d 13 C ratios are well dated by trilobite collections to the middle of the Steptoean Stage (Dunderbergia Zone) and occur during maximum regression associated with formation of the Sauk II- Sauk III subsequence boundary on the North American craton. Max- imum regression was marked by an influx of quartz sand into carbon- ate-platform settings in all three widely separated basins. In northern Utah, this quartz sand formed a thick sequence known as the Worm Creek Quartzite, which marks a conspicuous interruption of carbonate deposition during the Middle to Late Cambrian in the region. In west- ern Newfoundland, the thickness of the quartz sand unit is much re- duced but still marks a brief shutdown of the carbonate factory that is unique to the Cambrian shelf succession of the area. In the central Iowa area of the cratonic interior, an upward-shallowing carbonate succession culminates in cross-stratified trilobite grainstones at the peak of the SPICE in Dunderbergia Zone time, and the lowest point on the relative-sea-level curve is associated with the occurrence of coarse quartz sand derived from the encroaching shoreface. Although it is difficult to determine precisely the departure from baseline d 13 C that marks the beginning of the SPICE excursion in the stratigraphic successions analyzed, our results are consistent with a rise and subsequent fall in d 13 C tracking a major regressive-transgressive event recorded across northern Laurentia. The correlation of a major d 13 C excursion with regression is similar to that described for the Late Ordovician, for which the pattern has been attributed to either in- creased carbonate relative to terrigenous weathering rates as ice sheets covered up organic-matter-containing silicates at high latitudes or high productivity and organic-carbon burial driven by oceanic overturn. The lack of known Steptoean-age ice sheets that could have affected the ratio of carbonate to silicate weathering rates suggests that organic- carbon burial was the likely cause of the SPICE event. We suggest that increased weathering and erosion rates during relative sea-level fall (Sauk II-III) increased the burial fraction of organic carbon in an expanded region of fine-grained siliciclastic deposits in shelf and upper slope environments during the Steptoean.

Carbon isotope (δ13C) stratigraphy across the Silurian–Devonian transition in North America: evidence for a perturbation of the global carbon cycle

Carbon isotope (δ13C) analyses of marine carbonates spanning the Silurian–Devonian transition are compared from three richly fossiliferous, well-dated sequences in North America. The three sections, in the central Appalachian Mountains (West Virginia), Great Basin (Nevada), and the southern Mid-continent (Oklahoma), reveal positive δ13C shifts beginning in the late Pridoli and reaching peak values as heavy as +5.8‰ in the earliest Lochkovian following the first occurrence of the conodont species Icriodus woschmidti and the graptolite Monograptus uniformis. A positive shift in δ13C is also recorded at this time in Gondwanan regions, including the global stratotype section and point for the Silurian–Devonian boundary at Klonk in the Czech Republic, as well as in sections in the Carnic Alps of Austria (Cellon), and Queensland, Australia. The available data from Euramerica and Gondwana are consistent with a scenario linking seawater δ13C enrichment to a eustatic drop during the Silurian–Devonian transition. Seawater δ13C likely increased as a result of enhanced carbonate weathering during exposure and erosion of older Silurian platform deposits. In addition, the fall in sea level appears to have enhanced nutrient delivery to the oceans and triggered an increase in organic carbon burial rates at or near the Silurian–Devonian boundary, as indicated by organic-rich deposition in Gondwanan basins. In terms of its magnitude, the Silurian–Devonian δ13C excursion of ≥+5‰ appears to be among the largest well-documented events in the Paleozoic, comparable to the shifts in the Late Ordovician and Early Mississippian.