Geology topics

T. Evans

Publications and source records attributed to T. Evans.

2 recordsLinked to original sources

USGS2020

He-CO2-N2 isotope and relative abundance characterization of geothermal fluids from the Ethiopian Rift

We report He-CO 2 -N 2 isotopic and relative abundances in free gases and dissolved gas phase of geothermal fluids from the Ethiopian Rift. Fluid samples were collected from ~30 geothermal localities from three key regions throughout rifted and non-rifted areas of Ethiopia. The majority of samples, including off-rift samples, indicate a strong contribution of mantle-derived He-C-N to the fluid samples. Helium ( 3 He/ 4 He) and δ15N-(N 2 ) isotope anomalies are highest (> 15.9RA and > +5.0‰, respectively) at a single locality in south Afar (Sodere), but the maximum δ13C-(CO 2 ) (-0.78‰) is found east of Lake Shalla in the Lake District of the Main Ethiopian Rift. High 3 He/ 4 He values, consistent with mantle plume contributions, are also evident in fluids from the Lake District, where fluids from the Lake Shalla site extend up to 15.5RA. CO 2 / 3 He values span over four orders of magnitude while δ13C-(CO 2 ) values cluster mostly between mantle-like values of -4 and -7‰; only samples east of Lake Shalla display more positive values. Atmospheric-derived nitrogen has likely influenced a number of measured δ15N-(N 2 ) values but following a correction for atmospheric-contamination, the majority of samples reveal positive values (up to 6.5‰) which appear to be coupled to high 3 He/ 4 He values. In regions affected by upwelling mantle plumes, such high values have been interpreted to reflect deep mantle inputs of recycled nitrogen.

Ethiopian Rift↗

USGS2002

The polar bear management agreement for the southern Beaufort Sea: An evaluation of the first ten years of a unique conservation agreement

Polar bears ( Ursus maritimus ) of the southern Beaufort Sea population, distributed from approximately Icy Cape, west of Point Barrow, to Pearce Point, east of Paulatuk in Canada, are harvested by hunters from both countries. In Canada, quotas to control polar bear hunting have been in place, with periodic modifications, since 1968. In Alaska, passage of the United State Marine Mammal Protection Act (MMPA) of 1972 banned polar bear hunting unless done by Alaska Natives for subsistence. However, the MMPA placed no restrictions on numbers or composition of the subsistence hunt, leaving open the potential for an overharvest with no possible legal management response until the population was declared depleted. Recognizing that as a threat to the conservation of the shared polar bear population, the Inuvialuit Game Council from Canada and the North Slop Borough from Alaska negotiated and signed a user-to-user agreement, the Polar Bear Management Agreement for the Southern Beaufort Sea, in 1988. We reviewed the functioning of the agreement through its first 10 years and concluded that, overall, it has been successful because both the total harvest and the proportion of females in the harvest have been contained within sustainable limits. However, harvest monitoring needs to be improved in Alaska, and awareness of the need to prevent overharvest of females needs to be increased in both countries. This agreement is a useful model for other user-to-user conservation agreements.

Beaufort Sea↗