The An-Nafud sand sea formed during the Pleistocene and occupies approximately 60,200 km 2 of northern Saudi Arabia. The erg has regional significance within the North African – Arabian Pleistocene desert trend, recording aeolian system development and early hominin migrations, while the underlying geology represents the poorly-documented Ediacaran and Paleozoic history of the northern sector of the Arabian plate. Reflection seismic data and deep wells have been used to establish the first description of the 3D geometry of this erg and the tectonostratigraphy of the underlying bedrock geology. The total volume of the erg is calculated at 9,399 km 3 , equivalent to 17.4 trillion tons of sand. The erg infills a cuesta topography eroded into gently-dipping Paleozoic strata. A widespread lake deposit is interpreted within the erg from reflection seismic, and relationships between major units within the erg include Late Pleistocene lake beds with hominin footprints overlain by 100 m - scale megadunes. A subcrop map to the erg establishes continuity of the pre-Pleistocene formation boundaries that trend away from the erg margins. A system of extensional faults with up to 1 km throw mapped within the Paleozoic strata is part of a more extensive trend associated with Red Sea rifting. One fault appears to offset the base of the erg by 125 m and related seismicity may have impacted Pleistocene hominins. The top of Precambrian basement occurs at depths of up to 6 km, infilling Ediacaran paleotopography that causes the oldest Paleozoic strata to onlap and differentially compact over features interpreted as eroded, castellated Precambrian granitic plutons.
The geomorphic history of Grand Canyon, and especially Western Grand Canyon, remains unsettled. Groundwater speleothems, in particular cave mammillaries, in and around Grand Canyon that denote paleo-water-tables are among other dateable materials such as basalts and travertines that produce incision histories of the canyon. Due to their preservation in caves and their U-Pb dating relevance, cave mammillaries yield the existence of paleo-water-tables and thus incision rate results at higher elevations above the Colorado River compared to other materials. Near-water-table speleothemic material such as cave mammillary is likely common within the canyon region and it is not restricted to caves or carbonate rocks. Here we report two paleo-water-tables on the Hualapai Plateau that have ages of 21.51 ± 0.83 Ma and 18.57 ± 0.43 Ma using the isotope dilution U-Pb method, and 20.87 ± 0.55 Ma and 18.34 ± 0.93 Ma using the laser ablation U-Pb method. The timing of these Hualapai Plateau paleo-water-tables complements other age evidence on the plateau and introduces important constraints for discussing both the “young” and “old” Western Grand Canyon arguments.
Duricrusts cover more than 10% of Earth’s land surface. They armor cratonic landscapes and retard erosion, yet the diagenetic processes governing their internal evolution remain incompletely understood. We present an integrated micro-analytical study of duricrust diagenesis in the Kalahari Basin, southern Africa, combining cathodoluminescence petrography, electron microprobe elemental mapping, laser ablation ICP-MS, stable and radiogenic isotope geochemistry, and cosmogenic nuclide dating. Two sites on contrasting structural blocks flanking the Okavango Rift Zone, comprising predominantly calcareous duricrusts, reveal a six-facies paragenetic sequence spanning the Middle Pleistocene. The northern site (KPH) records open-system meteoric-phreatic cementation with depleted stable isotope values (δ¹³C: −8.2 to −6.8‰; δ¹⁸O: −9.0 to −7.1‰) and progressing radiogenic ⁸⁷Sr/⁸⁶Sr signatures indicating interaction with Archaean crystalline basement. The southern site (OKW) preserves an evaporative concentration sequence yielding carbonates with extreme strontium enrichment (median 6452 ppm) and high Mg/Ca ratios (median 0.61), followed by aggressive dissolution and replacement by authigenic Mg-silicates during humid intervals. Cosmogenic nuclide burial ages of 1.4–1.1 Ma for palustrine carbonates at KPH, capped by aeolian sand emplaced by 0.74 ± 0.14 Ma, and exposure ages of 145–352 ka of the granitoid-gneiss plateau at OKW, demonstrate that duricrust formation and modification in structural blocks situated less than 200 km apart record contrasting histories of accumulation, stripping, and re-cementation. Together, the chronological framework reveals that mineral recycling within duricrust profiles can proceed orders of magnitude faster than basin-average erosion. Within the limits of the two studied profiles, these results show that duricrusts represent dynamic recycling systems in which arid phases build reactive mineral reservoirs that are rapidly dissolved and reprecipitated during humid phases. Solute transport from structural highs to adjacent lows derives from various sources, through both surface and subsurface pathways in fractured host rocks, maintaining chemical reactivity within apparently stable landscapes. Cratonic landscape resilience thus arises from continuous internal reorganization rather than passive preservation.
Plio-Pleistocene marine sediments from Florida record glacial-interglacial sea level cycles and the evolution of shallow shelf ecosystems of the Florida Platform. These deposits also provide opportunities to reconstruct coastal ocean temperatures during past interglacial highstands and evaluate long term changes in West Atlantic oceanography. Here, we use carbonate clumped (Delta(47)) and bulk oxygen (delta O-18(carb)) isotope paleothermometry to test whether shallow marine temperatures during two Pleistocene highstands on the Florida Platform differed from those on the modern Atlantic coast of Florida. We first characterize the lithostratigraphy and biostratigraphy of a newly exposed section at Beeline Quarry in Orange County, east-central Florida that contains fossiliferous marine deposits of the upper Nashua Formation (Early Pleistocene) and a younger unit that is temporally equivalent to the lower Fort Thompson Formation (late Middle Pleistocene). Amino acid racemization data from the Fort Thompson-equivalent unit indicate deposition prior to MIS 5e (similar to 121 ka). Delta(47)-derived carbonate formation temperatures and water oxygen-isotope (delta O-18(water)) reconstructions are then presented from four fossil bivalve genera (Mulinia, Mercenaria, Dinocardium, and Chione). Reconstructed delta O-18(water) values are consistent with marginal-marine deposition and seasonally-targeted Delta(47) measurements in Chione exhibit subannual delta O-18(water) variations (>2 parts per thousand) consistent with freshwater influence. Unexpectedly, Delta(47) temperatures from Mercenaria, Dinocardium, and Chione in the Fort Thompson are 4-5 degrees C cooler than modern coastal waters. Although Mulinia appears to exhibit a genus-specific cold bias, it indicates that temperature conditions were similar (cooler-than-modern) during both the Nashua and Fort Thompson highstand intervals. Rather than reflecting regionally-cooler marine climates, we propose that these cool temperature signals most likely record the influence of localized hydrologic processes, potentially including submarine groundwater discharge or seasonal upwelling along the ancient East Florida Shelf.
Distinctive associations of microcrystalline thrombolites and fibrous aragonite cements in deep-marine Miocene New Zealand hydrocarbon seep carbonates capture snapshots of microbial resilience and adaptation to changing seepage intensity driven by hydrocarbon-rich fluid fluxes. These successions record repeated fracturing, sealing, and seepage recurrences, with thrombolites occurring within fractures and vugs and as coatings on worm tubes, intergrown with multiple generations of fibrous aragonite cements. Carbonate fabrics exhibit a wide range of carbon stable isotope values, indicating mixed thermogenic-biogenic hydrocarbon sources that sustained chemosynthesis-based communities in which sulfate-driven anaerobic oxidation of methane (SD-AOM) stimulated authigenic carbonate precipitation, whereas the oxygen isotope spread is consistent with kinetically controlled precipitation. These primary isotopic values were subsequently modified by burial diagenesis. Lipid biomarker assemblages dominated by strongly 13C-depleted archaeal and bacterial compounds, including diagnostic lipids of anaerobic methane-oxidizing archaea (ANME) and associated sulfate-reducing bacteria (SRB), together with additional 13C-depleted lipids derived from aerobic methanotrophs, demonstrate the operation of both anaerobic and aerobic methane oxidation. Rare earth element (REE) patterns in thrombolites and associated fibrous aragonite, characterized by fabric-dependent variations in REE contents, reveal a dynamic interplay among seawater ingress, microbial elemental scavenging and organic complexation, and diagenetic modification during late-stage diagenesis. Microbial growth and mineralization produced clotted, microcrystalline carbonate with thrombolite fabrics, expressed as both stratiform mat-like structures and digitate to bulbous bodies. These fabrics likely reflect adaptation to steep redox and fluid-chemical gradients and were subsequently overprinted during burial by neomorphic recrystallization and interaction with calcium-saturated fluids, producing fabric-modifying textures. This study provides a refined record of the interplay between microbial communities, advective-diffusive fluid systems, mineral precipitation, and diagenetic overprinting in deep-sea chemosynthesis-based ecosystems.
