SPICE Event Research Articles

Cascading oxygen loss shoreward in the oceans: Insights from the Cambrian SPICE event

Cascading oxygen loss shoreward in the oceans: Insights from the Cambrian SPICE event

Marine anoxia events across the upper Cambrian (stage 10) of eastern Gondwana: Implications from paleoenvironmental geochemical proxies

The uppermost Cambrian succession (Stage 10) of the Wa'ergang section (∼300 m of rhythmites, South China) records three globally correlated negative δ13Ccarb excursions (N1, N2, and the HERB- Helenmaria – Red Tops Boundary), overlying the SPICE event (Steptoean Positive Carbon Isotope Excursion), indicating perturbations in the global carbon cycle. The trigger and process of these events still remain elusive. The petrographic and chemical screening of the investigated carbonates proves well preservation of sedimentary fabrics and at least near-primary geochemical signatures. The paleoredox proxy (UEF/TOC, VEF/TOC, and MoEF/TOC) and δ15Norg profiles exhibit positive excursions that correlate with the negative δ13Ccarb excursions, indicating pulses of expanded oceanic anoxia potentially resulted from anoxic deep-water upwelling. The correlated negative shifts in TOC and paleoproductivity proxy (NiEF and CuEF) profiles suggest suppressed primary productivity, possibly by upwelling-induced oceanic toxic events, aligning with mass extinctions particularly during the HERB event. Independently, the paired δ13Ccarb and δ13Corg (Δ13C) variations reveal decreased biological fractionation that aligns with the δ13Ccarb excursions, implying pulses of declining atmospheric O2 levels, which are consistent with the suggested expanded oceanic anoxia and suppressed primary productivity. The peaks of the negative δ13Ccarb excursions are correlated with the base of upward-shallowing sequences, consistent with the evidence from TOC, δ13Corg, δ15Norg, and trace element proxies, support a scenario of upwelling deep toxic waters during sea-level rise that likely resulted in expanded oceanic anoxia, and ultimately led to the negative excursions in δ13Ccarb values and mass extinction.

The boundary between the Middle and Upper Cambrian strata in the Tarim Basin, NW China: Regional to global SPICE event correlation

The subdivision and correlation of the Cambrian System have long been controversial globally with multiple schemes. This is especially the case for the Tarim Basin, western China where regional and global correlations of the Middle-Upper Cambrian boundary are hampered by incomplete biostratigraphy. The Middle-Upper Cambrian boundary (T81) in the Tarim Basin is characterized by an abrupt switch of lithological and sedimentary facies, a sudden change of wireline log responses, and the contrasting seismic features, structural styles and carbonate platform architectures. The T81 boundary manifests in various forms and styles including syn-sedimentary unconformity, overlapping unconformity, tectonic unconformity, multi-stage superimposed unconformity, parallel unconformity and conformity. Each manifestation type of the T81 boundary was influenced by at least one of the factors including paleo-geomorphology, tectonic movement, eustacy, and climatic change. Based on the correlation between carbon isotopes and sedimentary characteristics in different regions globally, we suggest that the δ13C weak negative shift that follows the strongly positive shift of the Late Cambrian SPICE event can serve as a reliable isotope stratigraphic marker for identifying the T81 boundary of the western Tarim Platform. The top of the Awatag Formation (the top of the Middle Cambrian) in the western Tarim Platform should thus be correlated to the top of the Mohershan Formation in the eastern Tarim Basin, corresponding to the top of the Miaolingian Stage. The absence of the SPICE event in the western part of the Tarim Basin may have been caused by a regional depositional hiatus associated with the T81 boundary.

Spatio-temporal distribution of the Cambrian maceriate reefs across the North China Platform

Maceriate reefs, characterized by centimeter- to decimeter-scale branching structures, flourished during the Cambrian to Early Ordovician on Laurentia, and Gondwana (the North China Platform). Dendritic and columnar maceriate reefs were newly discovered from the northwestern and central parts of the North China Platform. The maceria structures are composed of sponges (e.g., keratosans and lithistids) and microbial components, including Girvanella, Renalcis, and Tarthinia, whereas the interstitial material consists of lime mud with bioclasts and intraclasts. The first appearance of maceriate reefs was diachronous across the North China Platform, younging from the northwest (Drumian Age, Miaolingian) to the southeast (Jiangshanian Age, Furongian). In general, thrombolites and dendrolites were dominant in the microbialite-dominated reefs of the Miaolingian and microbe–sponge maceriate reefs were dominant in the Furongian. This dramatic shift to microbe–metazoan reef assemblages resulted from a combination of factors associated with the Miaolingian–Furongian boundary interval, including global transgression and a shift from widespread anoxia of the SPICE event to more oxygenated post-SPICE conditions. The North China examples suggest that sea level and paleoenvironmental conditions may have played a major role in this transition and its timing.

Zinc isotopic evidence for enhanced continental weathering and organic carbon burial during the late Cambrian SPICE event

The globally recognized late Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE; ca. 497–494 Ma) is one of the most prominent carbon isotope excursions during the Phanerozoic eon, yet its primary cause remains debatable. To provide new insights into this issue, here we investigate zinc stable isotopes (expressed as δ66Zn) on two coeval carbonate successions spanning the Miaolingian-Furongian transition in North China. Similar stratigraphic δ66Zn trends are observed in the two sections, indicating a widespread perturbation to the marine Zn cycling. An abrupt decline of δ66Zn values by up to 0.4‰ and a concomitant increase of Zn/Ca ratios are observed at the pre-SPICE to the early SPICE interval, which can be best explained by the enhanced influx of isotopically light Zn from continental weathering. Enhanced continental weathering increased the inputs of terrestrial-sourced nutrients to the oceans, promoting the primary productivity and burial fluxes of organically bound Zn. At the middle stage of the SPICE, the coupled positive shifts of δ66Zn and δ13Ccarb indicate a phase of marine Zn cycling fluctuations driven by massive organic matter burial. Enhanced sequestration of organic carbon caused a rise in atmospheric oxygen levels, favoring the remobilization of previously buried Zn at shallower marine settings. With the development of muted continental weathering during the falling limb of the SPICE, the return of δ66Zn to initial background values marks the reestablishment of the stable seawater chemistry condition. Our results constrain the important roles of enhanced continental weathering and organic matter burial in micronutrient fluctuations during the SPICE, which were most likely the primary trigger for the SPICE.

Global and regional controls on carbon-sulfur isotope cycling during SPICE event in south China

Global and regional controls on carbon-sulfur isotope cycling during SPICE event in south China

Sedimentary mercury as a proxy for redox oscillations during the Cambrian SPICE event in western Newfoundland

The Steptoean Positive Isotopic Carbon Excursion, or SPICE event, was a globally recorded +2‰ to +6‰ shift in δ13C values during the later Cambrian (ca. 500 Ma). The excursion is associated with regional sea level change and trilobite extinctions documented from multiple paleocontinents, yet the mechanism(s) for these events and the excursion itself are not fully understood. Recent studies have suggested that the SPICE is accompanied by global changes in redox conditions, which may have served as an extinction driver. Here, we present new geochemical and mineralogical data from an outcrop of the SPICE located along the Port au Port Peninsula in western Newfoundland. We focus on paired sedimentary mercury and glauconite enrichments as a potential redox indicator in an effort to evaluate local redox conditions during the SPICE in Newfoundland and assess the validity of Hg as a redox proxy. These strata record small mercury enrichments (<20 ppb) relative to SPICE background, sometimes coupled with elevated glauconite abundance. This coupling occurs after the carbon isotope peak, suggesting that the environment during the falling limb of SPICE experienced redox oscillations. Findings from this study supplement existing global proxies by adding information on redox conditions during the SPICE in a shallow shelf environment. Further, the coupling of Hg enrichments with elevated glauconite provides some support for the use of sedimentary Hg as a redox indicator when enrichments are elevated compared to background.

Exotic facies episodes of a carbonate platform: implications for middle and late Cambrian ecosystems and impact of bioturbation in the Alborz Basin, Iran

Abstract The sedimentological and ichnological data of the Mila Group in the Alborz Basin, northern Iran indicate that the appearance of exotic facies and resurgence of microbialites resulted from the development of extraordinary palaeoceanographic conditions with widespread environmental stress and enhanced precipitation of CaCO 3 . Inhospitable environmental conditions in the Mila Group platform led to a considerable reduction in ichnodiversity, bioturbation intensity and depth and size of burrows, which led to periods of dominating calcimicrobe ecology. The development of exotic or anachronistic facies started around the Cambrian Stage 4–Wuliuan boundary, probably simultaneous with the SPICE event, whereas the Furongian exotic facies period probably took place simultaneous with the HERB event. Biotic and physico-chemical changes in the ocean during these events may have led as well to the virtual absence of the mixed layer and infaunal ecospace utilization in the deposits of the Mila Group. Nevertheless, the local presence of metazoan-microbial build-ups that established between the two suggested exotic facies developments indicates that these periods started at the Cambrian Stage 4–Wuliuan boundary interval, but a fluctuation of abnormal palaeoceanographic conditions and ambient normal marine conditions took place during the Miaolingian and Furongian in the Alborz Basin as a representative of the northern Gondwanan margin.

Carbon-isotope stratigraphy of the SPICE event (Upper Cambrian) in eastern Laurentia: implications for global correlation and a potential reference section

AbstractThe δ13C profile from the lower interval of the Martin Point section in western Newfoundland (Canada) spans the Upper Cambrian (uppermost Franconian – lowermost Trempealeauan). The investigated interval (∼110 m) is a part of the Green Point Formation of the Cow Head Group and consists of the upper part of the Tucker Cove Member (topmost part of the Shallow Bay Formation) and the lowermost part of the Martin Point Member (bottom of the Green Point Formation). It is formed of rhythmites of marine carbonates alternating with shales and minor conglomeratic interbeds. Multiscreening petrographic and geochemical techniques have been utilized to evaluate the preservation of the investigated lime mudstones. The δ13C and δ18O values of the sampled micrites (−4.8 ‰ to +1.0 ‰ VPDB and −8.2 ‰ to −5.3 ‰ VPDB, respectively) have insignificant correlation (R2= 0.01), as similarly do the δ13C values with their Sr counterparts (R2= 0.07), which supports the preservation of at least near-primary δ13C signatures that can be utilized to construct a reliable high-resolution carbon-isotope profile for global correlations. The δ13C profile exhibits two main negative excursions: a lower excursion (∼4 ‰) that reaches its maximum at the bottom of the section and an upper narrow excursion (∼6 ‰) immediately above the boundary of the Tucker Cove/Martin Point members (Shallow Bay Formation – Green Point Formation boundary). The lower excursion may be correlated with the global SPICE event, whereas the upper excursion may match with a post-SPICE event that has been also recognized in profiles of equivalent sections on different palaeocontinents.

Evidence of a large δ13Ccarb and δ13Corg depth gradient for deep-water anoxia during the late Cambrian SPICE event

Evidence of a large δ13Ccarb and δ13Corg depth gradient for deep-water anoxia during the late Cambrian SPICE event

Biostratigraphy and carbon and nitrogen geochemistry of the SPICE event in Cambrian low-grade metamorphic black shale, Southern Norway

We present new chemo- and biostratigraphical data from 90m of Alum Shale Formation stratigraphy from the middle Cambrian (Series 3) to the lowermost Ordovician (Tremadocian) at Krekling, southern Norway. The alum shale was deposited during a transgression covering parts of the Baltic craton, and contains up to 13.5wt.% total organic carbon (TOC) increasing up section through the Furongian. Pyrolysis confirms that the alum shale is affected by low grade metamorphism, either from a nearby pluton or during deep burial, where the remaining organic matter has lost most of the hydrocarbon productivity (S2=0–0.9mgHc/g rock) compared to equivalent formations in Sweden (TOC=8–22wt.%; S2=20–80). Still, the carbon isotopic trend (δ13Corg) shows that the positive carbon isotope excursion known as the SPICE event (Furongian) is preserved and that the carbon isotopes did not fractionate significantly while undergoing metamorphism. The δ13Corg increases steadily from the base of the section, from −30.8‰ to a maximum of −27.3‰, likely due to increasing atmospheric oxygen concentrations and cyanobacteria productivity. The heaviest values are recorded in an interval with abundant pyrrhotite nodules, suggesting that the organic productivity led to euxinic conditions, as shown from several other SPICE-sections worldwide. The evolution of bulk rock nitrogen isotopes (δ15N) shows positive correlations with δ13Corg and TOC, with an increase from −2 to +1.5‰. Before the onset of the SPICE, negative δ15N values indicate that atmospheric nitrogen was fixed in the shallow ocean. During the transition from low to high TOC shale at about 18m in the section, both the nitrogen content and the δ15N increase, but the δ15N is not high enough to imply water mass nitrogen reduction (denitrification) as a dominant process. Instead we suggest that the Krekling section was affected by sediment denitrification during SPICE, a process resulting in a ca. 1.5‰ post-depositional increase in δ15N. At the same time, in the upper parts of the water column, atmospheric nitrogen was still fixed by cyanobacteria. We conclude that the Cambrian ocean transgressed the Baltic craton and inhibited deep water upwelling, leaving geochemical signals characteristic of other greenhouse climates spanning the Phanerozoic.

Elucidating the relationship between the later Cambrian end-Marjuman extinctions and SPICE Event

The late Cambrian-early Ordovician transition contains several discrete marine extinction events. The first of these extinctions, the end-Marjuman, occurs in two phases and is thought to coincide with the beginning of the Steptoean Positive Carbon Isotope Excursion, or SPICE, a large excursion in the marine carbon isotope record that represents a large perturbation to the carbon cycle during this time. Additionally, the carbon isotope record from the Deadwood Formation in the Black Hills of South Dakota, USA, displays a small negative δ13C excursion near the end-Marjuman extinctions. Here we examine the carbon isotope stratigraphy of the Upper Cambrian portion of the Conasauga Group of the Southern Appalachians, USA, to determine the relative timing between the extinction events and changes in the carbon cycle represented by excursions within the carbon isotope record. Previous high-resolution biostratigraphic studies have identified a thick stratigraphic record of end-Marjuman extinctions within the Conasauga Group, making it an excellent target for a high-resolution chemostratigraphic study. In the Conasauga Group, there is no change in carbon isotope stratigraphy across the first phase of the end-Marjuman extinctions, suggesting no major change occurred in the carbon cycle during this time. Further, a negative δ13C excursion is absent in the Conasauga Group across the interval that contains the end-Marjuman extinctions. This suggests that the excursion in the Deadwood Formation is either a local oceanographic signal or a diagenetic feature. Finally, the onset of the SPICE occurs at the same stratigraphic point as the second phase of the end-Marjuman extinctions and at the appearance of a low diversity, potentially low oxygen tolerant, trilobite fauna. The stratigraphic positions of these biological and geochemical events suggest a role for marine anoxia in the second phase of the end-Marjuman extinctions.

The onset of the ‘Ordovician Plankton Revolution’ in the late Cambrian

The ‘Great Ordovician Biodiversification Event’ comprises the rapid diversification of marine organisms during the Ordovician Period. It is now clear that this adaptive radiation started for some organisms already in the Cambrian and continued for others beyond the end of the Ordovician, making the ‘Great Ordovician Biodiversification Event’ part of a long-term late Proterozoic and Early Palaeozoic radiation, that in part is expressed by the fossil record as the ‘Cambrian Explosion.’ A significant diversification of different groups of the plankton is observed in the late Cambrian–Early Ordovician interval, leading to the subsequent ‘Ordovician Plankton Revolution.’ The possible causes of this ‘plankton revolution’ are currently debated. They include changes in palaeoclimate, palaeogeography or tectonic and volcanic activity, as well as a modified nutrient supply. In this context, the Steptoean Positive Carbon Isotope Excursion δ13Ccarb (SPICE) event in the late Cambrian (Paibian Stage, Furongian Series) has been related to a major increase in atmospheric O2 (from 10–18% to some 20 – 29%) and to increased oceanic nutrient availability. Here we analyze the diversification of the planktonic groups during the late Cambrian and Early Ordovician, in particular in relation to the SPICE event. Our analyses include the changing diversities of the phytoplankton (acritarchs), diverse groups of zooplankton (e.g., radiolarians, graptolites, chitinozoans) and the switch to a planktonic mode of life of fossil groups (e.g., arthropods, molluscs) that were part of the Cambrian benthos and that later occupied pelagic niches. In addition, we focus also on data indicating evidence for a late Cambrian to Ordovician origin of planktotrophy in invertebrate larvae. It can be concluded that none of the diversifications of the different planktonic organisms can be related directly to the SPICE event. However, a long term (10–20millionyears) oxygenation pulse related to the SPICE event might have fuelled the explosion of phytoplankton diversity observed in the latest Cambrian–Early Ordovician that led to completely modified trophic structures permitting an increase in diversity and abundance of plankton-feeding groups during the Ordovician.

Uranium isotopes distinguish two geochemically distinct stages during the later Cambrian SPICE event

Anoxic marine zones were common in early Paleozoic oceans (542–400 Ma), and present a potential link to atmospheric pO2 via feedbacks linking global marine phosphorous recycling, primary production and organic carbon burial. Uranium (U) isotopes in carbonate rocks track the extent of ocean anoxia, whereas carbon (C) and sulfur (S) isotopes track the burial of organic carbon and pyrite sulfur (primary long-term sources of atmospheric oxygen). In combination, these proxies therefore reveal the comparative dynamics of ocean anoxia and oxygen liberation to the atmosphere over million-year time scales. Here we report high-precision uranium isotopic data in marine carbonates deposited during the Late Cambrian ‘SPICE’ event, at ca. 499 Ma, documenting a well-defined −0.18‰ negative δ238U excursion that occurs at the onset of the SPICE event's positive δ13C and δ34S excursions, but peaks (and tails off) before them. Dynamic modelling shows that the different response of the U reservoir cannot be attributed solely to differences in residence times or reservoir sizes – suggesting that two chemically distinct ocean states occurred within the SPICE event. The first ocean stage involved a global expansion of euxinic waters, triggering the spike in U burial, and peaking in conjunction with a well-known trilobite extinction event. During the second stage widespread euxinia waned, causing U removal to tail off, but enhanced organic carbon and pyrite burial continued, coinciding with evidence for severe sulfate depletion in the oceans (Gill et al., 2011). We discuss scenarios for how an interval of elevated pyrite and organic carbon burial could have been sustained without widespread euxinia in the water column (both non-sulfidic anoxia and/or a more oxygenated ocean state are possibilities). Either way, the SPICE event encompasses two different stages of elevated organic carbon and pyrite burial maintained by high nutrient fluxes to the ocean, and potentially sustained by internal marine geochemical feedbacks.

Integrated biostratigraphy and carbon isotopes from the Cambrian Tangwangzhai section, North China

The Tangwangzhai section, western Shandong Province, North China, the type section for the Cambrian Kushan and Chaomitian formations, yielded a diverse and relatively well-preserved conodont fauna, in which we recognize the Westergaardodina orygma, Westergaardodina matsushitai, Muellerodus? erectus, and Westergaardodina aff. fossa–Prooneotodus rotundatus zones of the North China conodont zonation. The Tangwangzhai conodont succession can be correlated not only with the polymerid trilobites occurring in the section but also with the conodont zones established for South China. The first occurrence of Furnishina longibasis and Furnishina quadrata in the upper part of the Westergaardodina matsushitai Zone allows the recognition of the base of the Paibian Stage and Furongian Series in the upper part of the Kushan Formation. The base of the Jiangshanian Stage, in the uppermost Muellerodus? erectus Zone, can be recognized by the presence of Westergaardodina cf. calix close to the base of the Chaomitian Formation. Chemostratigraphic analyses of the Tangwangzhai section show the onset of a positive carbon isotope excursion, referred to the SPICE event, in the upper part of the Kushan Formation at a level corresponding to the first occurrence of F. longibasis and F. quadrata. The base of the Jiangshanian Stage in the section is close to the demise of the SPICE positive excursion.

Oxygen as a Driver of Early Arthropod Micro-Benthos Evolution

BackgroundWe examine the physiological and lifestyle adaptations which facilitated the emergence of ostracods as the numerically dominant Phanerozoic bivalve arthropod micro-benthos.Methodology/Principal FindingsThe PO2 of modern normoxic seawater is 21 kPa (air-equilibrated water), a level that would cause cellular damage if found in the tissues of ostracods and much other marine fauna. The PO2 of most aquatic breathers at the cellular level is much lower, between 1 and 3 kPa. Ostracods avoid oxygen toxicity by migrating to waters which are hypoxic, or by developing metabolisms which generate high consumption of O2. Interrogation of the Cambrian record of bivalve arthropod micro-benthos suggests a strong control on ecosystem evolution exerted by changing seawater O2 levels. The PO2 of air-equilibrated Cambrian-seawater is predicted to have varied between 10 and 30 kPa. Three groups of marine shelf-dwelling bivalve arthropods adopted different responses to Cambrian seawater O2. Bradoriida evolved cardiovascular systems that favoured colonization of oxygenated marine waters. Their biodiversity declined during intervals associated with black shale deposition and marine shelf anoxia and their diversity may also have been curtailed by elevated late Cambrian (Furongian) oxygen-levels that increased the PO2 gradient between seawater and bradoriid tissues. Phosphatocopida responded to Cambrian anoxia differently, reaching their peak during widespread seabed dysoxia of the SPICE event. They lacked a cardiovascular system and appear to have been adapted to seawater hypoxia. As latest Cambrian marine shelf waters became well oxygenated, phosphatocopids went extinct. Changing seawater oxygen-levels and the demise of much of the seabed bradoriid micro-benthos favoured a third group of arthropod micro-benthos, the ostracods. These animals adopted lifestyles that made them tolerant of changes in seawater O2. Ostracods became the numerically dominant arthropod micro-benthos of the Phanerozoic.Conclusions/SignificanceOur work has implications from an evolutionary context for understanding how oxygen-level in marine ecosystems drives behaviour.

Parallel, high-resolution carbon and sulfur isotope records of the evolving Paleozoic marine sulfur reservoir

Links between the biogeochemical cycles of carbon and sulfur are expressed in the evolving stable isotope composition of the ocean. Carbonate rocks record the inorganic carbon isotope composition of the oceanic reservoir through geological time, along with the sulfate sulfur isotope composition preserved as carbonate-associated sulfate (CAS). An inverse relationship exists between the first-order carbon and sulfur records of the Paleozoic; however, the isotope curves also reveal higher-frequency variations superimposed on the first-order (10 7–10 8 yr) trend. Because of links between carbon and sulfur cycling, high-resolution, CAS-based S isotope data have the potential to shed essential light on the mechanisms behind carbon isotope excursions observed in the geologic record. Results from the Late Cambrian SPICE event at Shingle Pass, Nevada, show parallel positive C (5‰) and S (25‰) isotope excursions, likely recording a large-scale marine organic carbon burial event with sympathetic pyrite burial. Carbon and sulfur records from other events throughout the Paleozoic reveal different and evolving relationships. A Silurian–Devonian boundary section at Strait Creek, West Virginia, reveals a positive C excursion of 6‰ but with an enigmatic, apparently negative S excursion (15‰) antithetic to the C shift. By contrast, a Kinderhookian–Osagean (Early Mississippian) section in the Confusion Range, Utah, shows invariant S isotope ratios across a C isotope excursion of 6‰. The coupling of the high-resolution C–S isotope records may have weakened in the mid-to-late Paleozoic with decreasing sensitivity of the seawater S isotope reservoir to flux changes as the marine sulfate concentrations increased through the Paleozoic. The progressively decreasing isotopic variability of CAS across three positive carbon excursions suggests that the Paleozoic was transitional between a Proterozoic ocean with rapid isotopic variability and thus low sulfate concentration and the comparatively more stable isotopic properties of a relatively more sulfate-rich Mesozoic and Cenozoic ocean. Furthermore, the emergence of the terrestrial ecosystems during the late middle and late Paleozoic yielded new loci of organic burial that also likely played a central role in the decoupling carbon and sulfur isotope records. Organic carbon burial in these sulfate-limited environments occurred in the absence of significant pyrite burial.

The Late Cambrian Spice ( 13C) 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.