Reports: AC247836-AC2: Use of Geochemical Proxies to Evaluate Paleosalinity and Carbon Flux in the Bell Canyon Formation, Guadalupe Mountains West Texas

Paula J. Noble , University of Nevada (Reno)

We have finished our analysis of the middle Permian Lamar Limestone and Reef Trail Member of the Bell Canyon Formation using stable carbon and oxygen isotopes, elemental geochemistry, and organic biomarkers as a proxy for change in basin hydrography and paleoproductivity. As stated in previous reports, some of these changes appear to be basin-scale phenomena, but others, particularly in the Reef Trail Member, may be a global signal relating to the end-Guadalupian extinction.

Results reported previously show environmentally driven fluctuations in radiolarian faunas from both the Lamar Limestone and Reef Trail members. Furthermore, geochemical proxies such as TOC and stable carbon and oxygen isotopes show a strong positive correlation to the radiolarian shifts.

The descriptive radiolarian work is completed and was published in 2010. Geochemical work on the Lamar limestone was published in part in 2011, and 2 more papers have been submitted. Understanding the faunal and geochemical oscillations recognized in the Lamar Limestone were the subject of a PhD project completed this year and is described in more detail below. The isotope and organic biomarker geochemistry of the Reef Trail is the subject of a MS project and is also described below.

Lamar Limestone member. Analysis of major element chemistry from the carbonate fraction show that Ca/Mg ratios may be indicative of large-scale basin salinity shifts and show a positive correlation to large-scale shifts in oxygen isotope values, also interpreted as a driven by paleosalinity. Ratios of Ca to Ba and Sr do not show sensitivity as a paleosalinity proxy. Oxygen isotopes also record small lithology-dependant shifts and larger trends. The small shifts are interpreted as a diagenetic artifact between siliclastic rich beds and purer limestone beds, but the larger shifts appear to be best related to salinity changes caused by the degree of basin restriction with the open ocean. One such change occurs in the upper Lamar and lower Reef Trail. Additional isotope work was completed on a coeval section from the Apache Mountains and shows similar excursions occur basin-wide. The base of the Reef Trail Member and has previously been associated with a lowstand event. Finer-scale oscillations in radiolarian fauna from spumellarian- to Follicucullus-dominated faunas reported previously show a periodicity, and may be climatically related. They are strongly correlated with terrestrial input into the basin, particularly terrestrially-derived organic carbon, which is interpreted to have a stimulatory effect causing spumellarian blooms. Thus the smaller scale faunal oscillations appear to record a productivity signal.

Geochemical proxies to investigate environmentally-driven faunal changes in the Reef Trail Member. The biomarker data, collected from the upper units of the Reef Trail Member, show the following: the dibenzothiophene to phenanthrene ratio (DBT/PHEN), a proxy for source material, fluctuates between marine and terrigenous source material, with increased terrigenous source material at the Reef Trail-Castile contact. Compound-specific isotopes were also examined. δ13C(C17 alkane)/ δ13C(C27 alkane), which indicates marine versus terrestrial contribution, supports the DBT/PHEN data with increasing terrestrial contribution near the Reef Trail-Castile contact. The pristane to phytane ratio (Pr/Ph), a proxy for paleo-redox conditions, is anoxic at the top of the Reef Trail. The increasing terrestrial signature may be associated with marine regression, resulting in basin restriction accompanied by local anoxia or increased bacterial activity.  However, anoxia may be associated with global perturbations relating to the end-Guadalupian event, as anoxia has been documented in other end Guadalupian sections throughout the world. Additional stable isotopes from the Reef Trail Member and a comparable section in the Apache mountains were analyzed this year to fill in gaps and identified a lower excursion interval in the Reef Trail that corresponds to a faunal shift in the macrofossil community previously reported by other researchers. Work is in progress to evaluate whether this lower shift may be correlative to the globally recognized pre-Guadalupian extinction excursion, as opposed to the shift observed at the top of the formation.

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