Coupling of the sedimentary sulfur and carbon cycles; an improved model

132

104

The first terrestrialization of species that evolved from previously aquatic taxa was a seminal event in evolutionary history. For vertebrates, one of the most important terrestrialized groups, this event was interrupted by a time interval known as Romer's Gap, for which, until recently, few fossils were known. Here, we argue that geochronologic range data of terrestrial arthropods show a pattern similar to that of vertebrates. Thus, Romer's Gap is real, occupied an interval from 360 million years before present (MYBP) to 345 MYBP, and occurred when environmental conditions were unfavorable for air-breathing, terrestrial animals. These model results suggest that atmospheric oxygen levels were the major driver of successful terrestrialization, and a low-oxygen interval accounts for Romer's Gap. Results also show that terrestrialization among members of arthropod and vertebrate clades occurred in two distinct phases. The first phase was a 65-million-year ( (1) indicates that arthropods, with species interpreted as fully land-based before 400 million years before present (MYBP) (2), preceded vertebrates onto land by tens of millions of years. The first body fossils identified as stegocephalians (limbed vertebrates) are known from strata Ϸ370 MYBP, such that the appearance of the limb with digits, the loss of the opercular apparatus, and the development of other characters that subsequently allowed a terrestrial lifestyle, presumably occurred during the 25-My interval between 385 and 360 MYBP (3, 4). Most of our understanding about these crucial vertebrate events comes from only a few freshwater deposits from Euramerican localities, with outcrops in Greenland producing the most prolific stegocephalian remains. Elginerpeton and Obruchevichthys were first, between 385 and 375 MYBP, followed several million years later by a modest radiation that included Ventastega, Ichythostega, Acanthostega, Tulerpeton, Metaxygnathus, and Hynerpeton. Although most of these taxa possessed limbs (although this is not certain for Elginerpeton and Obruchevichthys), all of these taxa have been interpreted as fully aquatic, rather than terrestrial (5-7). Their respiratory systems are poorly known, but osteology suggests that they were able to derive some amount of O 2 from water instead of being entirely air breathing (8). These stegocephalians disappeared soon thereafter, followed rapidly by rare appearances of limbed vertebrates. This temporal gap has long been called Romer's Gap, and until recently it was unknown whether this hiatus was due to a combination of unfavorable taphonomic conditions and collection failure or whether it represented an interval of intrinsically low diversity and an abundance of limbed vertebrates. Romer's Gap separates occurrences of the earliest (Late Devonian) aquatic stegocephalians from a much larger adaptive radiation that included the first terrestrial vertebrates that commenced during the Visean stage of the Early Carboniferous. Although new collections have partly filled Romer's Gap (9), including the d...

Section: Methodsmentioning

confidence: 99%

Confirmation of Romer's Gap as a low oxygen interval constraining the timing of initial arthropod and vertebrate terrestrialization

Ward

Labandeira

Laurin

et al. 2006

132

104

“…2 and 3), which we assume to be tracking seawater sulfate and DIC, may at first blush seem contradictory because the burial rates of reduced carbon and sulfur are positively coupled in modern marine environments through BSR, which is fueled by organic matter and associated pyrite formation (18). Our understanding of their biogeochemical cycles, however, also allows for the possibility of an inverse isotopic relationship in the sulfate and DIC reservoirs (42)(43)(44). Indeed, there is a well-documented antithetic relationship between sulfate δ 34 S and carbonate δ 13 C values during some long-lived Phanerozoic carbon isotope excursions [e.g., during the Carboniferous (42,45) and the Phanerozoic, in general, on very long time scales (10 7 -to 10 8 -year time scales)].…”

Section: Sulfur Cycle During the Lomagundi Carbon Isotope Excursionmentioning

confidence: 99%

“…Indeed, there is a well-documented antithetic relationship between sulfate δ 34 S and carbonate δ 13 C values during some long-lived Phanerozoic carbon isotope excursions [e.g., during the Carboniferous (42,45) and the Phanerozoic, in general, on very long time scales (10 7 -to 10 8 -year time scales)]. This relationship has been linked to the mass balance between oxidized and reduced C and S compounds (18,43,(46)(47)(48) through the equation:…”

Section: Sulfur Cycle During the Lomagundi Carbon Isotope Excursionmentioning

confidence: 99%

Sulfur record of rising and falling marine oxygen and sulfate levels during the Lomagundi event

Planavsky

Bekker

Hofmann

et al. 2012

184

130

Carbonates from approximately 2.3-2.1 billion years ago show markedly positive δ 13 C values commonly reaching and sometimes exceeding þ10‰. Traditional interpretation of these positive δ 13 C values favors greatly enhanced organic carbon burial on a global scale, although other researchers have invoked widespread methanogenesis within the sediments. To resolve between these competing models and, more generally, among the mechanisms behind Earth's most dramatic carbon isotope event, we obtained coupled stable isotope data for carbonate carbon and carbonate-associated sulfate (CAS). CAS from the Lomagundi interval shows a narrow range of δ 34 S values and concentrations much like those of Phanerozoic and modern marine carbonate rocks. The δ 34 S values are a close match to those of coeval sulfate evaporites and likely reflect seawater composition. These observations are inconsistent with the idea of diagenetic carbonate formation in the methanic zone. Toward the end of the carbon isotope excursion there is an increase in the δ 34 S values of CAS. We propose that these trends in C and S isotope values track the isotopic evolution of seawater sulfate and reflect an increase in pyrite burial and a crash in the marine sulfate reservoir during ocean deoxygenation in the waning stages of the positive carbon isotope excursion.Lomagundi excursion | Great Oxidation Event | Precambrian

“…Indeed, the most comprehensive history of Phanerozoic oxygenation has been inferred from biogeochemical models. These models diverge, however, on their predictions for the Paleozoic, suggesting either low (15) or high levels of atmospheric oxygen (16,17). Here, we present an independent record of ocean oxygenation history derived from the isotopic composition and concentration of Mo in black shales.…”

mentioning

confidence: 95%

Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish

Dahl

Hammarlund

Anbar

et al. 2010

368

236

The evolution of Earth's biota is intimately linked to the oxygenation of the oceans and atmosphere. We use the isotopic composition and concentration of molybdenum (Mo) in sedimentary rocks to explore this relationship. Our results indicate two episodes of global ocean oxygenation. The first coincides with the emergence of the Ediacaran fauna, including large, motile bilaterian animals, ca. 550-560 million year ago (Ma), reinforcing previous geochemical indications that Earth surface oxygenation facilitated this radiation. The second, perhaps larger, oxygenation took place around 400 Ma, well after the initial rise of animals and, therefore, suggesting that early metazoans evolved in a relatively low oxygen environment. This later oxygenation correlates with the diversification of vascular plants, which likely contributed to increased oxygenation through the enhanced burial of organic carbon in sediments. It also correlates with a pronounced radiation of large predatory fish, animals with high oxygen demand. We thereby couple the redox history of the atmosphere and oceans to major events in animal evolution.T he concentration of O 2 in the Earth's atmosphere and oceans has increased over time from negligible levels early in Earth history to the 21% we have in the atmosphere today (1-3). Our understanding of this history is indirect, based mainly on a series of geochemical proxies reflecting chemical interactions between O 2 and other oxidation-reduction (redox) sensitive elements. These proxies include the isotopic compositions and concentrations of elements such as S, Fe, and Mo preserved in sedimentary rocks (3-7). Several of these proxies (4,5,8,9) indicate an increase in oceanic O 2 during the Ediacaran Period (635 to 542 million years ago, Ma), roughly synchronous with the emergence of large, motile bilaterian animals and, therefore, suggestive of a physiological link between Ediacaran evolution and environmental change. Despite this, the distribution of organic-rich shales (10, 11), the ratio of pyrite sulfur to organic carbon in shales (12), and modeling of the sulfur isotope record (13,14) all indicate that large tracts of subsurface ocean remained anoxic well into the early Phanerozoic Eon (the "age of visible animals," since 542 million years ago). Levels of ocean and atmospheric oxygenation, however, are unquantified from these proxies. Indeed, the most comprehensive history of Phanerozoic oxygenation has been inferred from biogeochemical models. These models diverge, however, on their predictions for the Paleozoic, suggesting either low (15) or high levels of atmospheric oxygen (16,17). Here, we present an independent record of ocean oxygenation history derived from the isotopic composition and concentration of Mo in black shales.The geochemical behavior of Mo is controlled by the relative availability of dissolved H 2 S and O 2 in the oceans. In oxic waters, Mo is soluble and exists as the molybdate anion, MoO 4 2− . In sulfidic waters, molybdate reacts with H 2 S to form particlereactive oxythiomo...