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We have continued our analysis of material collected from 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. In addition to basin-scale changes, we may be capturing a more global signal relating to the end-Guadalupian extinction at the very top of our Reef Trail section.

Results reported in previous years 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 this year in a special volume on the Guadalupian of the Delaware basin. Understanding the faunal and geochemical oscillations recognized in the Lamar Limestone are the subject of a PhD project and continuing analysis of these data is described below. The isotope and organic biomarker geochemistry of the Reef Trail is the subject of a MS project and progress on this project is also described below.

Lamar Limestone member. Analysis of a suite of geochemical proxies show that elemental ratios of Ca, Sr, and Mg are not sufficiently sensitive to be used as a paleosalinity proxy in this unit. No further work will be done along this avenue. Stable oxygen isotopes from inorganic carbonate seem to be the most sensitive paleosalinity proxy once diagenetic artifacts are removed from the dataset. Oxygen isotopes 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. 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. This year focused on analyzing the biomarker data, which 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 and δ13C(C17 alkane)/δ13C(C27 alkane), which indicates marine versus terrestrial contribution and 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.  However, anoxia may be associated with the end-Guadalupian event, because anoxia has been documented in other sections throughout the world leading up to the extinction.

 The final phase of investigation will be a comparative analysis of Reef Trail equivalent rocks in the Apache Mountains at the southern end of the Delaware basin. Stable isotope analysis of these rocks are under way to determine if the large shifts observed in the Guadalupe Mountains also occur to the south.