Evidence of a Radical PaleoOcean Chemistry Change Circa 545 Million Years Ago From Russia

Demise of Ediacaran dolomitic seas marks widespread biomineralization on the Siberian Platform

Authors:

Wood et al

Abstract:

The trigger for biomineralization of metazoans in the terminal Ediacaran, ca. 550 Ma, has been suggested to be the rise of oxygenation or an increase in seawater Ca concentration, but geochemical and fossil data have not been fully integrated to demonstrate cause and effect. Here we combine the record of macrofossils with early marine carbonate cement distribution within a relative depth framework for terminal Ediacaran to Cambrian successions on the eastern Siberian Platform, Russia, to interrogate the evolution of seawater chemistry and biotic response. Prior to ca. 545 Ma, the presence of early marine ferroan dolomite cement suggests dominantly ferruginous anoxic "aragonite-dolomite seas," with a very shallow oxic chemocline that supported mainly soft-bodied macrobiota. After ca. 545 Ma, marine cements changed to aragonite and/or high-Mg calcite, and this coincides with the appearance of widespread aragonite and high-Mg calcite skeletal metazoans, suggesting a profound change in seawater chemistry to "aragonite seas" with a deeper chemocline. By early Cambrian Stage 3, the first marine low-Mg calcite cements appear, coincident with the first low-Mg calcite metazoan skeletons, suggesting a further shift to "calcite seas". We suggest that this evolution of seawater chemistry was caused by enhanced continental denudation that increased the input of Ca into oceans so progressively lowering Mg/Ca, which, combined with more widespread oxic conditions, facilitated the rise of skeletal animals and in turn influenced the evolution of skeletal mineralogy.

How Salty Were the Seas of the Rhyacian PaleoProterozoic?

PIXE and microthermometric analyses of fluid inclusions in hydrothermal quartz from the 2.2 Ga Ongeluk Formation, South Africa: implications for ancient seawater salinity

Authors:

Saito et al

Abstract:

The Ongeluk Formation mainly consists of submarine volcanics (pillow lavas and sheet flows) composed of basaltic andesites that is between underlying and overlying glaciogenic deposits (i.e., the Makganyene diamictite and a dropstone layer at the base of the Hotazel Formation, respectively). The stratigraphic position of the Ongeluk Formation indicates that the Ongeluk volcanism occurred during a period of global glaciation. The Ongeluk volcanic rocks are host to subseafloor hydrothermal quartz deposits as drainage cavities and interpillow voids. The hydrothermal quartz contains many primary (Type 1) and secondary (Type 2) liquid-vapor fluid inclusions, as well as inclusions that are randomly distributed without a trace of secondary healed cracks (Type 3). All these fluid inclusions types were individually analyzed with microthermometry and particle-induced X-ray emission (PIXE) methods. The results show that Type 1 fluid inclusions are highly saline, whereas Type 2 fluid inclusions are relatively less saline. Type 3 fluid inclusions have bimodal peaks of salinity corresponding to those of Type 1 and Type 2 fluid inclusions and thus appear to represent mixtures between latter two inclusion types. Among the various fluid inclusion salinities of fluid inclusions, a wide range in Na/Ca values was identified in the high-salinity fluid inclusions (i.e., Type 1 and a subset of Type 3) which are thought to represent subseafloor fluids circulated by the Ongeluk submarine volcanism. The wide range in Na/Ca values can be explained by a mixing process between Na-rich and Ca-rich fluids associated with albitization of the host basaltic andesites. In the albitization process, Na is removed from the fluids and fixed by the host rocks; by contrast Ca is released from the host rocks into the fluids. PIXE analysis also showed two distinct trends (i.e., vertical and horizontal) on variation diagram of Ca/Cl versus Cu/Cl and positive correlations between Mn/Cl, Cu/Cl, Zn/Cl and Pb/Cl. These data are best explained by the presence of other mixing processes between; 1) a Na-rich, Ca- and Cu-poor fluid, 2) a Ca-rich, Na- and Cu-poor fluid, and 3) a Na- and Cu-rich, Ca-poor fluid. The Cu-, Mn-, Zn- and Pb-rich fluid likely represents a high-temperature hydrothermal fluid from a deep reaction zone in the Ongeluk subseafloor hydrothermal system. By contrast, the (1) Na- (and K-) rich, Ca- and Cu-poor endmember for the high-salinity primary inclusions is considered to represent the composition of 2.2 Ga Ongeluk seawater. We propose the estimation of 2.2 Ga Ongeluk seawater composition is > ∼2,237 mmol/kg of Na, 200–1,000 mmol/kg of K, < 135 mmol/kg of Ca, ∼3,230 mmol/kg of Cl (average values), and ∼400–500 of Cl/Br.

The minor sulfur isotope composition of Cretaceous and Cenozoic seawater sulfate

The minor sulfur isotope composition of Cretaceous and Cenozoic seawater sulfate

Authors:

Masterson et al

Abstract:

The last 125 million years captures major changes in the chemical composition of the ocean and associated geochemical and biogeochemical cycling. The sulfur isotopic composition of seawater sulfate, as proxied in marine barite, is one of the more perplexing geochemical records through this interval. Numerous analytical and geochemical modeling approaches have targeted this record. In this study we extend the empirical isotope record of seawater sulfate to therefore include the two minor sulfur isotopes, 33S and 36S. These data record a distribution of values around means of Δ33S and Δ36S of 0.043 ± 0.016‰ and -0.39 ± 0.15‰, which regardless of δ34S-based binning strategy, is consistent with a signal population of values throughout this interval. We demonstrate with simple box modeling that substantial changes in pyrite burial and evaporite sulfate weathering can be accommodated within the range of our observed isotopic values.

The Archean Nickel Famine Revisited

The Archean Nickel Famine Revisited

Authors:

Konhauser et al

Abstract:

Iron formations (IF) preserve a history of Precambrian oceanic elemental abundance that can be exploited to examine nutrient limitations on early biological productivity. However, in order for IF to be employed as paleomarine proxies, lumped-process distribution coefficients for the element of interest must be experimentally determined or assumed. This necessitates consideration of bulk ocean chemistry and which authigenic ferric iron minerals controlled the sorption reactions. It also requires an assessment of metal mobilization reactions that might have occurred in the water column during particle descent and during post-depositional burial. Here, we summarize recent developments pertaining to the interpretation and fidelity of the IF record in reconstructions of oceanic trace element evolution. Using an updated compilation, we reexamine and validate temporal trends previously reported for the nickel content in IF (see Konhauser et al., 2009). Finally, we reevaluate the consequences of methanogen Ni starvation in the context of evolving views of the Archean ocean-climate system and how the Ni famine may have ultimately facilitated the rise in atmospheric oxygen.

The Archean Banded Iron Formations of Krivoy Rog, Ukraine

Geochemistry of the Krivoy Rog Banded Iron Formation, Ukraine, and the impact of peak episodes of increased global magmatic activity on the trace element composition of Precambrian seawater

Authors:

Viehmann et al

Abstract:

Pure Superior-type Banded Iron Formation (BIF) samples from the Krivoy Rog Supergroup (Ukraine) are excellent archives of ambient Early Precambrian seawater. They show low concentrations of incompatible elements such as Zr, Hf, and Th, and shale-normalized Rare Earths and Yttrium (REYSN) patterns similar to those of modern seawater, i.e. heavy REYSN enriched patterns with positive LaSN, GdSN and YSN anomalies. Lack of CeSN and presence of positive EuSN anomalies indicate REY contributions to anoxic ferruginous seawater from high-temperature hydrothermal fluids.

The depositional age of the Krivoy Rog BIF is ill-defined, but a Late Archean to Paleoproterozoic age has been suggested based on U–Pb zircon ages for units stratigraphically above and below the BIF. We determined Sm–Nd isotopic compositions of pure and impure samples from the Krivoy Rog BIF, which yield an errorchron with an apparent age of 2406 ± 350 Ma (MSWD 15), that falls within this broad age range. All pure BIF samples show chondrite-normalized (subscript CN) REY patterns with strong positive EuCN anomalies that are typical for Archean but rather rare and much less pronounced in Proterozoic BIFs. Associated schists also show Archean – rather than post-Archean-style REY distributions. The REY geochemistry of both, chemical and epiclastic sediments, therefore, is more consistent with a Late Archean rather than a post-Archean depositional age of the Krivoy Rog Supergroup.

Initial ɛNd values of impure BIFs and of associated schist reveal variable contributions from TTGs less radiogenic in Nd and a more radiogenic component possibly comprised of basement amphibolites or mafic volcanics of the stratigraphically underlying New Krivoy Rog Group. The purest Krivoy Rog BIF, representing local Krivoy Rog seawater, displays an ɛNd2.60 Ga value of −2.3. This value is less radiogenic than impure Krivoy Rog BIFs or other near-contemporaneous Neoarchean pure chemical sediments. To preserve this specific local isotopic fingerprint in anoxic Archean seawater, the Krivoy Rog BIF must have been deposited in an isolated sea basin with limited exchange with ferruginous deep-waters of the open ocean.

A compilation of REY data for high-purity Precambrian BIFs reveals that EuCN/Eu*CN ratios of Precambrian seawater follow a general global evolution curve, that shows specific peaks which reflect times of increased high-temperature hydrothermal REY input into seawater. Following declining EuCN/EuCN ratios from the Eoarchean to the Mesoarchean, the ratios suddenly rise at 2.7 Ga and reach a maximum at 2.6 Ga, indicating an increased flux of high-temperature hydrothermal REY to Neoarchean seawater, which supports the hypothesis that times of widespread BIF deposition coincided with periods of intense submarine hydrothermal activity, probably triggered by major mantle plume events. This association is supported by a strong increase of the ɛNd(t) values of pure seawater archives at 2.7–2.6 Ga, which reflects an increased flux of mantle Nd into seawater. These results suggest that Eu-REY systematics (and potentially ɛNd systematics) are robust tools to indentify episodes of enhanced mantle plume activity.

Sea Water Chemistry Changes in the Shallow Water Ediacaran NeoProterozoic Ocean

Secular changes of water chemistry in shallow-water Ediacaran ocean: Evidence from carbonates at Xiaofenghe, Three Gorges area, Yangtze Platform, South China

Authors:

Hohl et al

Abstract:

Ediacaran carbonates from the shallow-water section near Xiaofenghe (Three Gorges area, Hubei Province) on the Yangtze Platform, South China have been studied to understand post Marinoan changes in seawater chemistry. Major and trace element abundances and Sr-Nd isotopic compositions were obtained on acetic acid leachates of carbonate sediments from both the Doushantuo Formation (Members D1-D4) and the overlying Dengying Formation. C and O isotopic compositions of these samples were analysed using the phosphoric acid method. With the exception of the D1 cap carbonates (87Sr/86Sr = 0.7088 to 0.713), most samples show low Mn/Sr and 87Sr/86Sr (0.7078 in D2-D4 and 0.709 in the Dengying Formation) similar to seawater values inferred from Ediacaran carbonates elsewhere. These data, together with the absence of a correlation between 87Sr/86Sr and δ18Ocarb, indicate minor diagenetic fluid overprinting. The initial ɛNd values of the carbonate leachates show a limited variation range from -4.3 to -7.5 with the highest values in D1. These data overlap with published data for Doushantuo sediments in South China, and possibly indicate an isotopically homogenous Nd source in the source area during most of the Ediacaran. D1 samples of cap carbonates display negative δ13Ccarb values of -2.0 to -3.6 (VPDB) and δ18Ocarb values ranging from -6.5 to -7.3 (VPDB), whereas the overlying D2 strata show positive δ13Ccarb values around 6. Carbonate rocks from D3 and D4 show a large variation of δ13Ccarb from -1.4 to 9.4, with some samples having unusually high δ13Ccarb values coupled to low TOC contents. Most carbonate leachates at the base of the Doushantuo display superchondritic Y/Ho and variable HREE enrichment (Pr/Yb less than 1), similar to modern seawater. This suggests a similar seawater chemistry as under modern open ocean conditions at the beginning of the Ediacaran, whereas lower Y/Ho in D3 correlate with flat REE + Y patterns, indicating a change to variable freshwater-seawater mixing and suppression of HREE/LREE fractionation as in modern estuaries. Carbonate rock samples from D4 are rich in clay minerals (up to 15%), of possible authigenic origin, such as nontronite and saponite. Formation of these clay minerals may only be possibly under extreme alkaline and saline conditions. The D4 carbonate leachates show the highest Y/Ho and lowest Ce/Ce* values and have unusually high δ 13Ccarb values, which may also support the conditions of extreme shallowing and perhaps temporarily restricted basins that led to a rise in salinity due to high evaporation rates. Ce anomalies in carbonate leachates show strong variation throughout the profile. A secular increase of seawater oxygenation through the Ediacaran is not obvious. However the most pronounced Ce anomalies can be found in the upper Doushantuo (Ce/Ce* as low as 0.64).

NeoTethys Seawater was Oxygenated, but had Increasingly and Rapidly Warming Temperatures Just Before the Permian Extinction

Neotethys seawater chemistry and temperature at the dawn of the end Permian mass extinction

Authors:

Garbelli et al

Abstract:

The end of the Permian was a time of great death and massive upheaval in the biosphere, atmosphere and hydrosphere. Over the last decades, many causes have been suggested to be responsible for that catastrophe such as global warming, anoxia and acidification. The Gyanyima limestone block was an open ocean seamount in the southern Neotethys at subtropical latitude, and it affords us insight into open-ocean oceanographic changes during the end of the Permian After careful screening using multiple tests, we reconstructed carbonate/seawater curves from the geochemical data stored in pristine brachiopod shell archives from the shallow water limestone of the Changhsingian Gyanyima Formation of Tibet. The reconstructed strontium isotope curve and data for the late Changhsingian is relatively invariant about 0.707013, but in the upper part of the succession the values become more radiogenic climaxing at about 0.707244. The 87Sr/86Sr curve and trend is similar to that observed for the Upper Permian succession in northern Italy, but dissimilar (less radiogenic) to whole rock results from Austria, Iran, China and Spitsbergen. The Ce/Ce* anomaly results. ranging from 0.310 to 0.577 for the brachiopods and from 0.237 to 0.655 for the coeval whole rock before the event, and of 0.276 for whole rock during the extinction event, suggest normal redox conditions. These Ce* values are typical of normal open-ocean oxic water quality conditions observed in modern and other ancient counterparts. The biota and Ce* information clearly discounts global anoxia as a primary cause for the end-Permian biotic crisis. Carbon isotopes from brachiopod shells and whole rock are relatively invariant for most of the latest Permian interval, which is in stark contrast to the distinct negative carbon isotope excursion observed near and about the event. Estimates of seawater temperature at shallow depth fluctuated from 22.2 to 29.0 °C up to unit 8-2, and then gradually rise from 29.7 °C in unit 8-13 to values exceeding 35 °C at a stratigraphic level about 120 ky before the Permian-Triassic boundary, and just before the onset of the extinction interval. This dramatic increase in seawater temperature has been observed in global successions from tropical to mid latitude and from restricted to open ocean localities (e.g., northern Italy, Iran). The brachiopod archive and its geochemical proxies from Tibet support the paradigm that global warming must have been an important factor of the biotic crisis for the terrestrial and marine faunas and floras of the late Paleozoic world.

A Record of PaleoArchean Sea Water Chemistry?!

Fluid inclusion analysis of silicified Palaeoarchaean oceanic crust – A record of Archaean seawater?

Authors:

Farber et al

Abstract:

In recent years, the role of Archaean seawater and hydrothermal fluid in the extensive silicification of Palaeoarchaean volcano-sedimentary successions has been a matter of considerable debate. In an attempt to constrain the conditions of silica precipitation, and the sources and chemical composition of the interacting fluids, we used fluid inclusion microthermometry, bulk crush-leach and oxygen isotope analyses of chert and quartz veins in silicified komatiites and sediments from the Barberton greenstone belt, South Africa. Chert vein margins consist of microcrystalline quartz and carbonaceous matter, whereas the vein centres are often filled with macrocrystalline quartz that contains abundant fluid inclusions.

Oxygen isotope ratios of vein chert and macrocrystalline quartz vary from 18 to 21‰, with the macrocrystalline quartz having slightly higher δ18O values (0.7 ± 0.3‰). The data are consistent with silica precipitation during low-temperature (≤100 °C) hydrothermal processes on the Archaean seafloor. Macrocrystalline quartz contains homogeneous 2-phase (L+V) inclusions at room temperature with a relatively constant vapour fraction. The inclusions have a salinity of 3–11 wt.% NaCl equiv. and homogenisation temperatures (Th) of 150–200 °C. Whereas some of the inclusions form intragranular fluid inclusion clusters that appear to be primary, other inclusions form transgranular fluid inclusion trails and are clearly secondary. Both types of inclusions share the same microthermometric characteristics, indicating that fluid entrapment occurred during a later metamorphic event and not near the seafloor. The event likely coincided with regional deformation and metamorphism at 3.23 Ga. Chlorite thermometry from vein host rocks reveals peak conditions of ∼257 ± 31 °C. In conjunction with microthermometry, the data correspond to a crustal depth of 3–6 km. A secondary origin of fluid inclusions is also consistent with the Cl/Br, Na/Cl and Na/K ratios of the macrocrystalline veins, which are similar to those found in metamorphic quartz veins. The fluid inclusions thus do not provide information on the conditions and temperatures during chert formation. While the fluids potentially carry the geochemical signature of modified Archaean seawater (i.e. hydrothermal fluid), characterised by low Cl/Br and Na/K ratios, and low Mg-contents, the fluid composition was likely modified during regional metamorphism.

Or was it Just Transitional? Marine Anoxic Episode Recorded in Cryogenian NeoProterozoic Canada

Chemostratigraphy of the Shaler Supergroup, Victoria Island, NW Canada: A record of ocean composition prior to the Cryogenian glaciations

Authors:

Thomson et al

Abstract:

A new δ13Ccarb curve combined with δ13Corg values is presented for the upper Shaler Supergroup (∼900 to ∼720 Ma), Amundsen Basin, northwestern Canada. The dataset fills gaps in the existing stratigraphic record and makes correlations with adjacent basins more robust. There is a pronounced negative δ13C excursion in the Wynniatt Formation that can be correlated with a putative worldwide negative carbon isotope excursion, namely the Bitter Springs stage. However, in the Amundsen Basin, the δ13Ccarb excursion drops to anomalously negative values (-14‰), which we attribute to local overprints wherein isotopically light carbon in pore waters, released by oxidation of methane and organic matter during sulphate and iron reduction, was incorporated into authigenic carbonate cement. We document basin euxinia and anoxia during the same time interval using a multi-proxy approach; specifically, Fe-speciation and redox-sensitive trace metal data. Patterns of pronounced enrichment in Mo, V, and U concentrations in euxinic black shales suggest that the Bitter Springs stage was a transitional period in Earth's redox evolution, from the more reduced global oceans during the mid-Proterozoic to the more oxygenated oceans during the Phanerozoic.

Enceladus' Great Subterranean Soda Seas From Serpentization

The pH of Enceladus' ocean

Authors:

Glein et al

Abstract:

Observational data from the Cassini spacecraft are used to obtain a chemical model of ocean water on Enceladus. The model indicates that Enceladus' ocean is a Na-Cl-CO3 solution with an alkaline pH of ~11-12. The dominance of aqueous NaCl is a feature that Enceladus' ocean shares with terrestrial seawater, but the ubiquity of dissolved Na2CO3 suggests that soda lakes are more analogous to the Enceladus ocean. The high pH implies that the hydroxide ion should be relatively abundant, while divalent metals should be present at low concentrations owing to buffering by clays and carbonates on the ocean floor. The high pH is interpreted to be a key consequence of serpentinization of chondritic rock, as predicted by prior geochemical reaction path models; although degassing of CO2 from the ocean may also play a role depending on the efficiency of mixing processes in the ocean. Serpentinization leads to the generation of H2, a geochemical fuel that can support both abiotic and biological synthesis of organic molecules such as those that have been detected in Enceladus' plume. Serpentinization and H2 generation should have occurred on Enceladus, like on the parent bodies of aqueously altered meteorites; but it is unknown whether these critical processes are still taking place, or if Enceladus' rocky core has been completely altered by past hydrothermal activity. The high pH also suggests that the delivery of oxidants from the surface to the ocean has not been significant, and the rocky core did not experience partial melting and igneous differentiation. On the other hand, the pH is compatible with life as we know it; life on Earth may have begun under similar conditions, and serpentinites on Earth support microbial communities that are centered on H2 that is provided by water-rock reactions.

 

Declinate in Oceanic Sulfate During Calymmian MesoProterozoic

Decline in oceanic sulfate levels during the early Mesoproterozoic

Authors:

Luo et al

Abstract:

Multiple-sulfur isotope compositions (32S, 33S, 34S and 36S) were analyzed for paired carbonate-associated sulfate (CAS) and disseminated pyrite (PY) from the ∼1.6-Ga Gaoyuzhuang Formation of the North China Craton to reconstruct the history of sulfate levels in Proterozoic oceans. The 200-m-thick study interval yielded relatively constant values for δ34SCAS (13.0 ± 1.8‰), δ34SPY (8.0 ± 2.3‰), and Δ34SCAS-PY (∼5‰), as well as relatively constant Δ33S (0 ± 0.05‰) and Δ36S (0.35 ± 0.15‰) for both CAS and pyrite. Limited variation in δ34SPY and slightly lower Δ33S of pyrite relative to CAS suggest water-column precipitation of pyrite. Limited fractionation of sulfur during microbial sulfate reduction (as documented by Δ34SCAS-PY) implies low seawater sulfate concentrations in the early Mesoproterozoic ocean. We quantitatively constrained paleo-seawater [SO42−] using a novel modeling approach based on measured values of Δ34SCAS-PY and ∂δ34SCAS/∂t(max). For the study unit, Δ34SCAS-PY is 5.4 ± 1.4‰ (n = 17), and ∂δ34SCAS/∂t(max) is 6.8-34‰ Myr−1 based on sedimentation rates of 30-150 m Myr−1. These data indicate early Mesoproterozoic seawater [SO42−] of ∼ < 0.1 to 0.35 mM (with a maximum possible concentration of 1.8 mM), a range that is lower and more tightly constrained than earlier estimates for the Mesoproterozoic. Compilation of published data suggests that low seawater sulfate concentrations began about ∼1.7 Ga and persisted until at least the mid-Mesoproterozoic (∼1.4 Ga), documenting a distinct early Mesoproterozoic perturbation in ocean chemistry that may have been related to a decline in atmospheric pO2 after Great Oxidation Event I.

Wild Speculation: Could a Phorphorus Crisis Have Ended the Francevillian Biota?

Iron oxides, divalent cations, silica, and the early earth phosphorus crisis

Authors:

Jones et al

Abstract:

As a nutrient required for growth, phosphorus regulates the activity of life in the oceans. Iron oxides sorb phosphorus from seawater, and through the Archean and early Proterozoic Eons, massive quantities of iron oxides precipitated from the oceans, producing a record of seawater chemistry that is preserved as banded iron formations (BIFs) today. Here we show that Ca2+, Mg2+, and silica in seawater control phosphorus sorption onto iron oxides, influencing the record of seawater phosphorus preserved in BIFs. Using a model for seawater cation chemistry through time, combined with the phosphorus and silica content of BIFs, we estimate that seawater in the Archean and early Proterozoic Eons likely contained 0.04–0.13 μM phosphorus, on average. These phosphorus limiting conditions could have favored primary production through photoferrotrophy at the expense of oxygenic photosynthesis until upwelling waters shifted from phosphorus to iron limiting.

Seawater Chemistry Measured From Cryogenian NeoProterozoic Australia

Analyses of fluid inclusions in Neoproterozoic marine halite provide oldest measurement of seawater chemistry

Authors:

Spear et al

Abstract:

We analyzed primary fluid inclusions in halite from marine evaporites in the ca. 830 Ma Browne Formation of the Officer Basin in Western Australia using the cryogenic scanning electron microscopy– energy dispersive X-ray spectrometry analysis technique. The concentrations of the major ions, except K+ and possibly SO42–, fall within the range of Phanerozoic seawater. This is the first direct measurement of the composition of mid-Neoproterozoic seawater, and extends present-day knowledge of seawater chemistry by ∼300 m.y. Our estimates suggest that mid-Neoproterozoic marine sulfate concentrations were lower (∼90%) than modern values. By the terminal Neoproterozoic, fluid inclusions in halite and evaporite mineralogy indicate seawater sulfate levels rose significantly, to 50%–80% of modern concentrations, which parallels increases in atmospheric and oceanic oxygen.