Exploring the Stable Isotope Record of Lake Carpenter: A Lacustrine Sequence in the Aptian–Albian Cretaceous Cedar Mountain Formation, Utah, USA

Abstract:

The Cedar Mountain Formation (CMF) represents the earliest deposition of terrestrial

Cretaceous strata in the USA, recording significant changes in biota and climate. Understanding

these transitions requires improved time constraints and high-resolution proxy records. Here we

present new δ¹³C (organic carbon & carbonate) chemostratigraphic record of a lacustrine

sequence in a locality named “Lake Carpenter”, near Moab, Utah. Lake Carpenter (LC)

comprises interbedded limestone and mudstone units of the Ruby Ranch Member of the CMF.

Results of the chemostratigraphy are constrained by detrital zircons from the section allowing

correlation of the chemostratigraphy to the carbon isotope segments C9 to C11 (Bralower et al.,

1999) spanning the Late Aptian to Early Albian, and supported by previous litho- and

chemostratigraphic work in the CMF. δ¹³Corg values show a pronounced negative stepped

excursion, of -6‰ with values reaching -32.3 ‰ occurring in conjunction with an increase in

TOC. This negative excursion is followed by a positive recovery, with values of ~-25‰ and

relatively low TOC. δ¹³Ccarb records positive values, up to +8‰, in the lowermost part of the

section (< ~7m) followed by a decrease to ~-7 ‰. δ¹⁸Ocarb over this interval records values

between -2 and -4‰ followed by a decrease to ~-7‰. The lowermost portion of the LC section

is indicative of relatively deep lacustrine environment in which organic carbon burial influenced

the δ¹³C of dissolved inorganic carbon (DIC) in the lake. This lower δ¹³C of DIC may be due to

increased upwelling and/or turnover and recycling of organic carbon in the lake. Variability of

δ¹³Ccarb and δ¹⁸Ocarb values may reflect changes in water supply to the lake, or climatic variability

resulting in the lake drying out. δ¹³Corg values may be affected by local lake dynamics, including

variations in organic carbon storage and changes in algal productivity, perhaps also indicative of

changes in nutrient availability due to increased run off and/or fluctuations in atmospheric

carbon particularly light δ¹³Corg. Both climatic and hydrological variation in the section may be

due to the rise of the Sevier Mountains resulting in an orographic impact on the lake either as a

result of increase in high altitude run off, or changes in the amount of precipitation.