Benoit Beauchamp

Growth and demise of Permian biogenic chert along northwest Pangea: evidence for end-Permian collapse of thermohaline circulation

Abstract The Permian Chert Event (PCE) was a 30 Ma long episode of unusual chert accumulation along the northwest margin of Pangea, and possibly worldwide. The onset of the PCE occurred at about the Sakmarian–Artinskian boundary in the Sverdrup Basin, Canadian Arctic, where it coincides with a maximum flooding event, the ending of high-frequency/high-amplitude shelf cyclicity, the onset of massive biogenic chert deposition in deep-water distal areas, and a long-term shift from warm- to cool-water carbonate sedimentation in shallow-water proximal areas. A similar and coeval shift is observed from the Barents Sea to the northwestern USA. A landward and southward expansion of silica factories occurred during the Middle and Late Permian at which time warm-water carbonate producers disappeared completely from the northwest margin of Pangea. Biotically impoverished and increasingly narrow cold-water carbonate factories (characterised by non-cemented bioclasts of sponges, bryozoans, echinoderms and brachiopods) were then progressively replaced by silica factories. By Late Permian time, little carbonate sediments accumulated in the Barents Sea and in the Sverdrup Basin, where the deep- to shallow-water sedimentary spectrum was occupied by siliceous sponge spicules. By that time, biogenic silica sedimentation was common throughout the world. Silica factories collapsed in the Late Permian, abruptly bringing the PCE to an end. In northwest Pangea, the end-Permian collapse of the PCE was associated with a major transgression and with a return to much warmer oceanic and continental climatic conditions. Chert deposition resumed in the distal oceanic areas during the early Middle Triassic (Anisian) after a 8–10 Ma interruption (Early Triassic Chert Gap). The conditions necessary for the onset, expansion and zenith of the PCE were provided by the thermohaline circulation of nutrient-rich cold waters along the northwestern and western margin of Pangea, and possibly throughout the world oceans. These conditions provided an efficient transportation mechanism that constantly replenished the supply of silica in the area, created a nutrient- and oxygen-rich environment favouring siliceous biogenic productivity, established cold sea-floor conditions, hindering silica dissolution, while increasing calcium carbonate solubility, and provided conditions adverse to organic and inorganic carbonate production. The northwest margin of Pangea was, for nearly 30 Ma, bathed by cold waters presumably derived from the seasonal melting of northern sea ice, the assumed engine for thermohaline circulation. This process started near the Sakmarian–Artinskian boundary, intensified throughout Middle and Late Permian time and ceased suddenly in latest Permian time. It led to oceanic conditions much colder than normally expected from the palaeolatitudes, and the influence of cold northerly-derived water was felt as far south southern Nevada. The demise of silica factories was caused by the rapid breakdown of these conditions and the establishment of a much warmer marine environment accompanied by sluggish circulation and perhaps a reduced input of dissolved silica to the ocean. Complete thawing of northern sea ice would have ended thermohaline circulation and led to warm and sluggish oceanic conditions inimical to the production, accumulation and preservation of biogenic silica.

Significance of Aragonite Cements Around Cretaceous Marine Methane Seeps

ABSTRACT Detailed petrography and geochemistry of carbonate precipitates in Cretaceous cold seep mounds from the Canadian Arctic show spectacular early diagenetic products: some still-preserved splays and isopachous layers of fine, acicular aragonite, and large botryoids and crusts of low-magnesium calcite showing unusual entanglement of former fibrous calcite and aragonite. The latter mineralogy is suggested by clear, flat-terminated cathodoluminescence patterns interpreted as ancient crystal growth steps, and the former by rhombohedral terminations. The early cement phases very likely precipitated in cold Arctic water dominated by bicarbonates derived from bacterially oxidized methane: these cements have d13C values around -44. and d18O values of 1.8 to 0.1 PDB. Coexistence of calcite and aragonite early cements in the Cretaceous seep mounds is unusual, because precipitation occurred in high-latitude, cold-water settings, and during a so-called calcite sea mode. As in modern marine hydrocarbon seeps, the chemistry of the Cretaceous system was apparently controlled by chemosynthetic bacterial activity, resulting in high aHCO3-, that promoted precipitation of carbonates. We suggest that, locally, fluctuations in aHCO3-/aSO42- resulted in oscillating aragonite or calcite supersaturation, and hence, controlled the mineralogy of the early precipitates.

Latest Permian mercury anomalies

A sedimentary record from the Buchanan Lake section, Canadian High Arctic, shows anomalous high levels of mercury (Hg) during the latest Permian extinction (LPE) on northwest Pangea. Significant influx of Hg progressively overwhelmed the marine system. Major disruption of the organic matter–mediated Hg drawdown process resulted in accumulation of dissolved Hg to maximum levels at the LPE boundary, affecting an already stressed global ecosystem. A switch to euxinic marine conditions at the LPE boundary led to chemical drawdown of Hg sulfides, as marked by progressive Hg mitigation. This allowed self-recovery from toxic Hg conditions, and ultimately led to reestablishment of the internal Hg–organic matter drawdown process. We hypothesize that anomalous Hg levels may be attributed to the significant natural atmospheric emissions caused by catastrophic Siberian Traps volcanic eruptions.

Recurrent Early Triassic ocean anoxia

The Early Triassic record, from the Smithian stratotype, shows that the organic carbon isotope record from northwest Pangea closely corresponds to major fluctuations in the inorganic carbon records from the Tethys, indicating truly global perturbations of the carbon cycle occurred during this time. Geochemical proxies for anoxia are strongly correlated with carbon isotopes, whereby negative shifts in δ 13 C org are associated with shifts to more anoxic to euxinic conditions, and positive shifts are related to return to more oxic conditions. Rather than by a delayed or prolonged recovery, the Early Triassic is better characterized by a series of aborted biotic recoveries related to shifts back to ocean anoxia, potentially driven by recurrent volcanism.

Sedimentary Cover of the Canadian Shield through Mesozoic Time Reflected by Nd Isotopic and Geochemical Results for the Sverdrup Basin, Arctic Canada

The Sverdrup Basin of the Canadian Arctic Islands contains a sedimentary record, with only short breaks, from Early Carboniferous to Late Cretaceous time and can be used to document the nature of sediments delivered from northern Canada and Greenland. Sm‐Nd isotopic analysis of 72 sedimentary rock samples from the Sverdrup Basin, coupled with trace element characterization, shows that for most of Carboniferous to Late Cretaceous time, the sediment supply in the northern part of North America was dominated by a single broad provenance; 56 of the 72 samples lie squarely within the Nd isotopic evolution of a clastic sedimentary cover delivered to the region following 450–350 Ma Caledonian and Franklinian mountain building in Greenland and the Canadian Arctic Islands. Cratonic Shield sources in Greenland and Canada are hardly evident in the record, and significant contributions to the sediment budget from any source other than the post‐mid‐Paleozoic orogenic cover occurred only during four relatively short periods. First, during Carboniferous time, pre–Late Ordovician rocks of the Franklinian orogen contributed to alluvial clastic rocks in small rift basins in northern Ellesmere Island. Second, during Early Cretaceous time, Shield basement contributed to more widespread deltaic deposits in central and southern Ellesmere Island. Third, minor volcanic contributions to much of the basin occurred during Late Triassic–earliest Jurassic time and also, fourth, during Late Cretaceous time. Sedimentary materials from Caledonian and Franklinian mountains dominated the provenance of the continental and continent‐margin sedimentary system for at least 370 m.yr., a period of time extending far beyond the existence of the mountains themselves. This dominance was achieved by recycling of widespread Middle and Upper Devonian strata into Mesozoic units in the Canadian Arctic and Cordillera. We assess the extent to which the results call for cover of the Greenland‐Canadian Shield from 450 to 80 Ma and conclude that while much of the Shield was probably covered by Ordovician to Middle Devonian carbonate units, the northerly derived Upper Devonian clastic sedimentary rocks probably covered about one‐half of the Shield in its western and northern portions. This cover was progressively removed through Mesozoic time.

Supraglacial sulfur springs and associated biological activity in the Canadian high arctic-signs of life beneath the ice.

Unique springs, discharging from the surface of an arctic glacier, release H(2)S and deposit native sulfur, gypsum, and calcite. The presence of sulfur in three oxidation states indicates a complex series of redox reactions. Physical and chemical conditions of the spring water and surrounding environment, as well as mineralogical and isotopic signatures, suggest biologically mediated reactions. Cell counts and DNA analyses confirm bacteria are present in the spring system, and a limited number of sequenced isolates suggests that complex communities of bacteria live within the glacial system.

Cretaceous Cold-Seep Communities and Methane-Derived Carbonates in the Canadian Arctic

Lower Cretaceous cold-seep fossil assemblages have been found in the Canadian Arctic Archipelago. Serpulid worm tubes and bivalves are most abundant in these communities; in contrast, fossils are scarce in the surrounding strata. The fossils are contained in an isotopically light (δ13C = -25 to -50 per mil) carbonate rock groundmass that is interpreted to have formed from bacterial oxidation of methane. The rocks were deposited at intermediate depth (≤400 meters) in a cold marine environment; nearby normal faults may have provided a conduit for seeping methane and hydrogen sulfide needed to fuel chemosynthetic bacteria, and in turn, the higher life forms.

Upper carboniferous to upper permian 13C-enriched primary carbonates in the sverdrup basin, Canadian arctic: Comparisons to coeval Western North American Ocean Margins

Abstract Carbon isotopes analyses performed on more than 250 limestone samples from thirteen late Paleozoic autochthonous formations of western and Arctic Canada demonstrate that: (1) For a given locality, the δ 13 C-values are uniform and there is no temporal increase in δ 13 C during the whole Late Carboniferous (Pennsylvanian), Early Permian and basal Late Permian (latest Permian strata being absent). (2) There is a well-defined latitudinal trend showing that for any specific time interval, extremely high δ 13 C-values are encountered in the Sverdrup Basin ( +5 to + 7%), intermediate but still high values in northern Yukon Territory ( +3 to +5%), and low values in east-central and southeastern British Columbia ( +1 to +3%). A model is proposed whereby oceanic stagnation and thermohaline stratification prevailed in the Sverdrup Basin. As a result, large amounts of 12 C-enriched organic matter were preserved in central parts of the basin. As the organic matter accumulated, residual dissolved carbonate species became enriched in 13 C. Additional enrichment was also promoted by the escape of CO, from evaporating brines following intermittent closures of the Sverdrup Basin early during the Late Carboniferous. Since the Sverdrup sea was connected to the Paleopacific ocean during most of its history, it acted as a source of 13 C-enriched surface waters which were then spread southward by wind-driven surficial currents. With this model, the trend of decreasing δ 13 C in a southerly direction is explained by increasing dilution with waters of the Paleopacific along the western margin of North America.

Molybdenum isotopic evidence for oxic marine conditions during the latest Permian extinction

The latest Permian extinction (LPE), ca. 252 Ma, represents the most severe extinction event in Earths history. The cause is still debated, but widespread marine anoxic to euxinic (H2S rich) conditions, from deep to shallow water environments, are commonly suggested. As a proxy for marine oxygen levels, we analyzed δ98/95Mo of two LPE sections that represent a gradient in water depth on the northwest margin of Pangea. Results from deep-water slope environments show a large shift in δ98/95Mo values from −2.02・to +2.23・at the extinction horizon, consistent with onset of euxinic conditions. In contrast, sub-storm wave base shelf environments show little change in the molybdenum isotopic composition (−1.34・to +0.05・, indicating ongoing oxic conditions across the LPE. These results indicate that areas of the continental shelf of northwest Pangea underwent mass extinction under oxic conditions throughout the LPE event, and that shallow-water anoxia was therefore not a global phenomenon.

Mercury anomalies associated with three extinction events (Capitanian Crisis, Latest Permian Extinction and the Smithian/Spathian Extinction) in NW Pangea

Strata of Permian – Early Triassic age that include a record of three major extinction events (Capitanian Crisis, Latest Permian Extinction and the Smithian/Spathian Extinction) were examined at the Festningen section, Spitsbergen. Over the c . 12 Ma record examined, mercury in the sediments shows relatively constant background values of 0.005–0.010 μg g –1 . However, there are notable spikes in Hg concentration over an order of magnitude above background associated with the three extinctions. The Hg/total organic carbon (TOC) ratio shows similar large spikes, indicating that they represent a true increase in Hg loading to the environment. We argue that these represent Hg loading events associated with enhanced Hg emissions from large igneous province (LIP) events that are synchronous with the extinctions. The Hg anomalies are consistent across the NW margin of Pangea, indicating that widespread mercury loading occurred. While this provides utility as a chemostratigraphic marker the Hg spikes may also indicate loading of toxic metals to the environment, a contributing cause to the mass extinction events.