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1. A window to the Early Cretaceous North American Climate and Environment: The ‘Lake Carpenter’ lacustrine strata of the Cedar Mountain Formation

   Suarez, M.B. ; Al-Suwaidi, A. ; Kirkland, J.I. ; Snell, K. ; Moller, A. ; McLean, N. ; Suarez, C.A. ; Montgomery, E. ; Paige, M. ( January 2023 , The Anatomical record Supplement)

   The Cedar Mountain Formation is thought to span a significant portion of the lower Cretaceous and the base of the upper Cretaceous (Valanginian to Cenomanian). As such, the Cedar Mountain Formation is important for understanding the transition of terrestrial ecosystems from those characterized by pre-angiosperm ecosystems of the Jurassic to the angiosperm-dominated ecosystems that characterized the height of dinosaur diversity in the later part of the Cretaceous. Lacustrine strata offer unique opportunities to shed light on environmental and climate conditions of the past. This study presents results from a multi-proxy study of lacustrine strata in the Cedar Mountain Formation termed “Lake Carpenter.” The sequence of strata is about ~30 m thick and located near Arches National Park. The lower ~7 m is characterized by dark organic-rich mudstones, shales, and tan limestones and dolostone. The middle portion between about 7 and 25m consists of more massive carbonate-rich strata with abundant aquatic fossils including ostracodes, charophytes, and fish scales. The upper portion to about 30 m consists of green to tan mudstones with carbonate nodules and increases in siliciclastic content. Carbonate mineralogies include calcite, high-magnesium calcite, and dolomite (including dolomicrites) based on XRD analyses. To put the lacustrine sequence into stratigraphic context, bulk organic C isotope values were utilized to construct a chemostratigraphic record. The carbon isotope values range from -32.3‰ to -21.1‰ vs. VPDB. Zircons from four suspected volcanic ash layers were analyzed for U-Pb using LA-ICP-MS. One of these produced concordant Cretaceous dates. The youngest zircons from this sample was analyzed using CA-ID-TIMS and produced a date of 115.65 ± 0.18 Ma. Based on the chemostratigraphic record and the U-Pb date, the deposition of the lacustrine sequence occurs in the mid to late Aptian and spans a time that is thought to have coincided with a cold snap based on marine records. Carbonate analyses of the carbonates within the lacustrine sequence ranges from -9.2‰ to +5.4‰ vs. VPDB for carbon and -9.3 to -0.3‰ vs. VPDB for oxygen. Overall, carbonate isotope data is positively covariant and along with the minerology, seems to suggest that the lake was a closed-basin, alkaline lake and would have likely experience significant evaporation. To investigate paleotemperature, selected samples were analyzed for clumped isotope values (47) to determine temperature of formation. Preliminary temperature estimates of calcite formation range from 27°C to 41°C. Estimates for dolomite range from 19°C to 21°C. Lacustrine carbonate formation typically is biased toward spring and summer and as such some of these temperatures (particularly the values for dolomites) seem slightly lower than expected for a greenhouse climate but may be consistent with a “cold-snap” during the late Aptian. Palustrine carbonates from the type section of the Ruby Ranch Member range 19.8°C to 44.5°C (Suarez et al. 2021) and suggests the lacustrine strata records a similar range in temperatures during the Aptian Stage in this part of North America. REFERENCES CITED: Suarez, MB, Knight, J, Snell, KE, Ludvigson, GA, Kirkland, JI, Murphy, L 2020. Multiproxy paleoclimate estimates of the continental Cretaceous Ruby Ranch Member of the Cedar Mountain Formation. In: Bojar, A-V, Pelc., A, Lecuyer, C, editors. Stable Isotopes Studies of Water Cycle and Terrestrial Environments. Geol Soc, London, Spec Pub, 507: https://doi.org/10.1144/SP507-2020-85 KEYWORDS: Early Cretaceous, lacustrine, stable isotopes, paleoclimate Presentation Mode: Invited Speaker 

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2. Frenchman Mountain Dolostone: A new formation of the Cambrian Tonto Group, Grand Canyon and Basin and Range, USA

   https://doi.org/10.1130/GES02514.1  

   Rowland, Stephen M. ; Korolev, Slava ; Hagadorn, James W. ; Ghosh, Kaushik ( March 2023 , Geosphere)

   We describe, interpret, and establish a stratotype for the Frenchman Mountain Dolostone (FMD), a new Cambrian stratigraphic unit that records key global geochemical and climate signals and is well exposed throughout the Grand Canyon and central Basin and Range, USA. This flat-topped carbonate platform deposit is the uppermost unit of the Tonto Group, replacing the informally named “undifferentiated dolomites.” The unit records two global chemostratigraphic events—the Drumian Carbon Isotope Excursion (DICE), when δ13Ccarb (refers to “marine carbonate rocks”) values in the FMD dropped to −2.7‰, and the Steptoean Positive Carbon Isotope Excursion (SPICE), when the values rose to +3.5‰. The formation consists of eight lithofacies deposited in shallow subtidal to peritidal paleoenvironments. At its stratotype at Frenchman Mountain, Nevada, the FMD is 371 m thick. Integration of regional trilobite biostratigraphy and geochronology with new stratigraphy and sedimentology of the FMD, together with new δ13Ccarb chemostratigraphy for the entire Cambrian succession at Frenchman Mountain, illustrates that the FMD spans ~7.2 m.y., from Miaolingian (lower Drumian, Bolaspidella Zone) to Furongian (Paibian, Dicanthopyge Zone) time. To the west, the unit correlates with most of the Banded Mountain Member of the ~1100-m-thick Bonanza King Formation. To the east, at Grand Canyon’s Palisades of the Desert, the FMD thins to 8 m due to pre–Middle Devonian erosion that cut progressively deeper cratonward. Portions of the FMD display visually striking, meter-scale couplets of alternating dark- and light-colored peritidal facies, while other portions consist of thick intervals of a single peritidal or shallow subtidal facies. Statistical analysis of the succession of strata in the stratotype section, involving Markov order and runs order analyses, yields no evidence of cyclicity or other forms of order. Autocyclic processes provide the simplest mechanism to have generated the succession of facies observed in the FMD. 

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3. Environmental and trilobite diversity changes during the middle-late Cambrian SPICE event

   https://doi.org/10.1130/B36421.1  

   Zhang, Lei ; Algeo, Thomas J. ; Zhao, Laishi ; Dahl, Tais W. ; Chen, Zhong-Qiang ; Zhang, Zihu ; Poulton, Simon W. ; Hughes, Nigel C. ; Gou, Xueqing ; Li, Chao ( May 2023 , Geological Society of America Bulletin)

   The Steptoean Positive Carbon Isotope Excursion (SPICE) event at ca. 497−494 Ma was a major carbon-cycle perturbation of the late Cambrian that coincided with rapid diversity changes among trilobites. Several scenarios (e.g., climatic/oceanic cooling and seawater anoxia) have been proposed to account for an extinction of trilobites at the onset of SPICE, but the exact mechanism remains unclear. Here, we present a chemostratigraphic study of carbonate carbon and carbonate-associated sulfate sulfur isotopes (δ13Ccarb and δ34SCAS) and elemental redox proxies (UEF, MoEF, and Corg/P), augmented by secular trilobite diversity data, from both upper slope (Wangcun) and lower slope (Duibian) successions from the Jiangnan Slope, South China, spanning the Drumian to lower Jiangshanian. Redox data indicate locally/regionally well-oxygenated conditions throughout the SPICE event in both study sections except for low-oxygen (hypoxic) conditions within the rising limb of the SPICE (early-middle Paibian) at Duibian. As in coeval sections globally, the reported δ13Ccarb and δ34SCAS profiles exhibit first-order coupling throughout the SPICE event, reflecting co-burial of organic matter and pyrite controlled by globally integrated marine productivity, organic preservation rates, and shelf hypoxia. Increasing δ34SCAS in the “Early SPICE” interval (late Guzhangian) suggests that significant environmental change (e.g., global-oceanic hypoxia) was under way before the global carbon cycle was markedly affected. Assessment of trilobite range data within a high-resolution biostratigraphic framework for the middle-late Cambrian facilitated re-evaluation of the relationship of the SPICE to contemporaneous biodiversity changes. Trilobite diversity in South China declined during the Early SPICE (corresponding to the End-Marjuman Biomere Extinction, or EMBE, of Laurentia) and at the termination of the SPICE (corresponding to the End-Steptoean Biomere Extinction, or ESBE, of Laurentia), consistent with biotic patterns from other cratons. We infer that oxygen minimum zone and/or shelf hypoxia expanded as a result of locally enhanced productivity due to intensified upwelling following climatic cooling, and that expanded hypoxia played a major role in the EMBE at the onset of SPICE. During the SPICE event, global-ocean ventilation promoted marine biotic recovery, but termination of SPICE-related cooling in the late Paibian may have reduced global-ocean circulation, triggering further redox changes that precipitated the ESBE. Major changes in both marine environmental conditions and trilobite diversity during the late Guzhangian demonstrate that the SPICE event began earlier than the Guzhangian-Paibian boundary, as previously proposed. 

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4. Upper Windermere Supergroup and the transition from rifting to continent-margin sedimentation, Nadaleen River area, northern Canadian Cordillera

   https://doi.org/10.1130/B32039.1  

   Moynihan, David P. ; Strauss, Justin V. ; Nelson, Lyle L. ; Padget, Colin D. ( March 2019 , GSA Bulletin)

   Abstract Neoproterozoic–Cambrian rocks of the Windermere Supergroup and overlying units record the breakup of Rodinia and formation of the northwestern Laurentian ancestral continental margin. Understanding the nature and timing of this transition has been hampered by difficulty correlating poorly dated sedimentary successions from contrasting depositional settings across Mesozoic structures. Here we present new litho- and chemo-stratigraphic data from a Cryogenian–lower Cambrian succession in east-central Yukon (Canada), establish correlations between proximal and distal parts of the upper Windermere Supergroup and related strata in the northern Canadian Cordillera, and consider implications for the formation of the northwestern Laurentian margin. The newly defined Nadaleen Formation hosts the first appearance of Ediacaran macrofossils, while the overlying Gametrail Formation features a large negative carbon isotope anomaly with δ13Ccarb values as low as –13‰ that correlates with the globally developed Shuram-Wonoka anomaly. We also define the Rackla Group, which includes the youngest (Ediacaran) portions of the Windermere Supergroup in the northern Cordillera. The top of the Windermere Supergroup is marked by an unconformity above the Risky Formation that passes into a correlative conformity in the Nadaleen River area. This surface has been interpreted to mark the top of the rift-related succession, but we draw attention to evidence for tectonic instability through the early-middle Cambrian and argue that the transition from rifting to post-rift thermal subsidence is marked by a widespread unconformity that underlies upper Cambrian carbonate rocks. This is younger than the interpreted age of the rift to post-rift transition elsewhere along the ancestral western Laurentian continental margin. 

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5. The Combined Effects of Ocean Acidification and Respiration on Habitat Suitability for Marine Calcifiers Along the West Coast of North America

   https://doi.org/10.1029/2023JC019892  

   Feely, Richard A. ; Carter, Brendan R. ; Alin, Simone R. ; Greeley, Dana ; Bednaršek, Nina ( April 2024 , Journal of Geophysical Research: Oceans)

   The California Current Ecosystem (CCE) is a natural laboratory for studying the chemical and ecological impacts of ocean acidification. Biogeochemical variability in the region is due primarily to wind‐driven near‐shore upwelling of cold waters that are rich in re‐mineralized carbon and poor in oxygen. The coastal regions are exposed to surface waters with increasing concentrations of anthropogenic CO₂(C_anth) from exchanges with the atmosphere and the shoreward transport and mixing of upwelled water. The upwelling drives intense cycling of organic matter that is created through photosynthesis in the surface ocean and degraded through biological respiration in subsurface habitats. We used an extended multiple linear‐regression approach to determine the spatial and temporal concentrations of C_anthand respired carbon (C_bio) in the CCE based on cruise data from 2007, 2011, 2012, 2013, 2016, and 2021. Over the region, the C_anthaccumulation rate increased from 0.8 ± 0.1 μmol kg⁻¹ yr⁻¹in the northern latitudes to 1.1 ± 0.1 μmol kg⁻¹ yr⁻¹further south. The rates decreased to values of about ∼0.3 μmol kg⁻¹ yr⁻¹at depths near 300 m. These accumulation rates at the surface correspond to total pH decreases that averaged about 0.002 yr^‐1; whereas, decreases in aragonite saturation state ranged from 0.006 to 0.011 yr^‐1. The impact of the C_anthuptake was to decrease the amount of oxygen consumption required to cross critical biological thresholds (i.e., calcification, dissolution) for marine calcifiers and are significantly lower in the recent cruises than in the pre‐industrial period because of the addition of C_anth.

    

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