The Early Cretaceous (145–100 Ma) was characterized by long-term greenhouse climates, with a reduced equatorial to polar temperature gradient, although an increasingly large body of evidence suggests that this period was punctuated by episodic global “cold snaps.” Understanding climate dynamics during this high-atmospheric CO2 period of Earth’s history may have significant impact on how we understand climatic feedbacks and predict future global climate changes under an anthropogenically-driven high-pCO2 atmosphere. This study utilizes facies analysis to constrain the paleobathymetry of Lower Cretaceous glendonites—a pseudomorph after ikaite, a mineral that forms naturally at 7 °C or lower—from two paleo-high-latitude (60–70°N) sites in Svalbard, Arctic Norway, to infer global climatic changes during the Early Cretaceous. The original ikaite formed in the offshore transition zone of a shallow marine shelf at water depths of <100 m, suggesting mean annual water temperatures of ≤7 °C at these depths at 60–70°N. We correlate glendonite-bearing horizons from Lower Cretaceous successions around the globe using carbon isotope stratigraphy, in conjunction with the pre-existing biostratigraphic framework, in order to infer northern hemispheric to global climatic cooling. A distinct interval of glendonites in the Northern Hemisphere, from sites >60°N, spans the late Berriasian to earliest Barremian (at least 8.6 m.y.), significantly prolonging the duration of the previously hypothesized Valanginian cold snap (associated with the “Weissert Event”). Widespread glendonites occur again in late Aptian and extend to the early Albian, in both hemispheres, corroborating other proxy evidence for late Aptian cooling. The glendonites from Svalbard suggest that Cretaceous cold episodes were characterized with high latitude (>60°N) shallow water temperatures that are consistent with the existence of a small northern polar ice cap at this time.

Glendonites, pseudomorphs after marine sedimentary ikaite, are found throughout the Lower Cretaceous succession of Svalbard. Existing models for the ikaite-to-glendonite transformation do not explain the different petrological fabrics observed in the glendonites of Lower Cretaceous Svalbard. This study presents an improved model for the formation of these glendonites, based on petrographic and geochemical observations, and published work on ikaite breakdown. We show that for the glendonites of Lower Cretaceous Svalbard, methane is unlikely to be the sole or indeed main driver behind their formation, and present an improved model for their formation that accounts for the varied petrographic fabrics observed in these particular glendonites. Coupled with our new model, stable isotope data demonstrate why bulk samples of ancient glendonite cannot be used for palaeotemperature reconstructions.

Significant changes in global climate and carbon cycling occurred during the Early Cretaceous. This study examines the expression of such climatic events in high-latitude Svalbard together with the stratigraphic utility of carbon-isotope stratigraphies. Isotopic analysis of fossil wood fragments (from the Rurikfjellet, Helvetiafjellet, and Carolinefjellet formations, Festningen, Spitsbergen) record a distinctive pattern including a negative isotope excursion preceding a positive event, correlatable with the global early Aptian isotope event. Our carbon-isotope profile improves the stratigraphic correlation and relative dating of the succession. We show that the upper part of the Helvetia fjellet Formation was deposited during the early Aptian, and not the late Barremian, as previously thought. Furthermore, we estimate an age for the abrupt contact of the Rurikfjellet Formation with the overlying Helvetiafjellet Formation (associated with a pulse of igneous activity) to be ca. 129 Ma or ca. 124 Ma, depending on which age model for the Early Cretaceous is used. The well-known dinosaur footprints of the Helvetiafjellet Formation at Fest ningen are constrained to the middle Barremian and, coupled with floral data, support a warm late Barremian prior to the Aptian carbon-isotope event. The appearance of glendonites at 655 m in the Carolinefjellet Formation is consistent with global cooling in the late Aptian-early Albian.

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Improved understanding of mouth bar morphodynamics, and the resulting stratigraphic architectures, is important for predicting the loci of deposition of different sediment fractions, coastal geomorphic change and heterogeneity in mouth bar reservoirs. Facies and architectural analysis of exceptionally well-exposed shallow water (ca 5 m depth) mouth bars and associated distributaries, from the Xert Formation (Lower Cretaceous), of the Maestrat Basin (east-central Spain), reveal that they grew via a succession of repeated autogenic cycles. An initial mouth bar accretion element forms after avulsion of a distributary into shallow standing water. Turbulent expansion of the fluvial jet and high bed friction results in rapid flow deceleration, and deposition of sediment in an aggradational to expansional bar-form. Vertical bar growth causes flattening and acceleration of the jet. The accelerated flow scours channels on the bar top, which focuses further expansion of the mouth bar at individual loci where the channels break through the front of the mouth bar. Here, new mouth bar accretion elements form, downlapping and onlapping against a readily recognizable surface of mouth bar reorganization. Vertical growth of the new mouth bar accretion elements causes flattening and re-acceleration of the jet, leading to channelization, and initiation of the next generation of mouth bar accretion elements. Thus the mouth bar grows, until bed-friction effects cause backwater deceleration and superelevation of flow in the feeding distributary. Within-channel sedimentation, choking and upstream avulsion of the feeding channel, results in mouth bar abandonment. In this study, mouth bars are formed of at least two to three accretion elements, before abandonment happened. The results of this study contrast with the notion that mouth bars form by simple vertical aggradation and radial expansion. However the architecture and facies distributions of shallow water mouth bars are a predictable product of intrinsic processes that operate to deposit them.