Paleoenvironmental change recorded in submarine fans: the Eocene-Oligocene climate transition in the Alpine foreland basin

The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifying the transition from Paleocene greenhouse to Oligocene icehouse conditions. Preservation of the sedimentary signal of such an environmental change is most likely in net-depositional environments, such as submarine fans, which are the terminal parts of sedimentary systems. Here, using sedimentological and stable isotope data from the Alpine foreland basin, we assess whether this major climatic transition influenced the stratigraphic evolution of submarine fans. Results indicate that submarine fan retreat in the Alpine foreland basin corresponds with positive δ13C excursions related to major global perturbations of the carbon cycle and cooling in the earliest Oligocene. Submarine fan retreat is suggested to be influenced by this cooling through enhanced aridity and reduced subaerial runoff from the Corsica-Sardinia hinterland. The influence of aridity was periodically overwhelmed by local environmental factors, such as hinterland uplift, which increased sediment supply to deep-water during arid periods. These results highlight that: 1) hinterland climate may play a greater role than sea-level in dictating sediment supply to deep-water and, 2) submarine fan evolution occurs through a complex interplay between climate, eustasy and tectonics, which makes robust interpretations of paleoenvironmental change from their stratigraphic record, without multi-proxy records, difficult.

New approaches to dating intermittently varved sediment, Columbine lake, Colorado, USA

Annually laminated lake sediment can track paleoenvironmental change at high-resolution where alternative archives are often not available. However, information about both paleoenvironmental change and chronology are often affected by indistinct and intermittent varves. We present an approach that overcomes these and other obstacles by using a quantitative varve quality index combined with a multi-core, multi-observer Bayesian varve sedimentation model that quantifies realistic under- and over-counting uncertainties while integrating information from radiometric measurements (210Pb, 137Cs, and 14C) into the chronology. We demonstrate this approach on thin sections of indistinct and intermittently varved sequences from alpine Columbine Lake, Colorado. The integrated model indicates 3137 (95 percentile highest density probability range: 2753–3375) varve years with a cumulative posterior distribution of counting uncertainties of −13/+7 % indicative of systematic observer undercounting. The sedimentary features of the thin and complex varves shift through time, from normally graded couplets to couplets interrupted with coarser sub-laminae, to inversely graded couplets. We interpret the normal grading couplets as spring nival discharge followed by winter settling, the coarser sub-laminae as high rainfall events, and the inverse grading as hyperpycnal flows and/or pulses of dust related to human impact changing the varve formation mechanism. Our novel approach provides a realistic constraint on sedimentation rates and quantifies uncertainty in varve counts by quantifying over- and under-counting uncertainties related to observer bias and the quality and variability of the sediment appearance. The approach permits the construction of a varve chronology and sedimentation rates for sites with intermittent or indistinct varves, which are likely more prevalent than sequences with distinct varves, and thus, expands the possibilities of reconstructing past environmental change with high resolution.

Supplemental Material: U-Pb zircon geochronology and depositional age models for the Upper Triassic Chinle Formation (Petrified Forest National Park, Arizona, USA): Implications for Late Triassic paleoecological and paleoenvironmental change

Assessing stratigraphic uncertainties between published dates and dates from CPCP 1A and two data tables.

Supplemental Material: U-Pb zircon geochronology and depositional age models for the Upper Triassic Chinle Formation (Petrified Forest National Park, Arizona, USA): Implications for Late Triassic paleoecological and paleoenvironmental change

Assessing stratigraphic uncertainties between published dates and dates from CPCP 1A and two data tables.

Supplementary Material: U-Pb zircon geochronology and depositional age models for the Upper Triassic Chinle Formation (Petrified Forest National Park, Arizona, USA): Implications for Late Triassic paleoecological and paleoenvironmental change

Assessing stratigraphic uncertainties between published dates and dates from CPCP 1A.