34 research outputs found
Methane sources in gas hydrate-bearing cold-seeps : evidence from radiocarbon and stable isotopes
Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 115 (2009): 102-109, doi:10.1016/j.marchem.2009.07.001.Fossil methane from the large and dynamic marine gas hydrate reservoir has the potential to influence oceanic and atmospheric carbon pools. However, natural radiocarbon (14C) measurements of gas hydrate methane have been extremely limited, and their use as a source and process indicator has not yet been systematically established. In this study, gas hydrate-bound and dissolved methane recovered from six geologically and geographically distinct high-gas-flux cold seeps was found to be 98 to 100% fossil based on its 14C content. Given this prevalence of fossil methane and the small contribution of gas hydrate (≤1%) to the present-day atmospheric methane flux, non-fossil contributions of gas hydrate methane to the atmosphere are not likely to be quantitatively significant. This conclusion is consistent with contemporary atmospheric methane budget calculations.
In combination with δ13C- and δD-methane measurements, we also determine the extent to which the low, but detectable, amounts of 14C (~ 1-2 percent modern carbon, pMC) in methane from two cold seeps might reflect in situ production from near-seafloor sediment organic carbon (SOC). A 14C mass balance approach using fossil methane and 14C-enriched SOC suggests that as much as 8 to 29% of hydrate-associated methane carbon may originate from SOC contained within the upper 6 meters of sediment. These findings validate the assumption of a predominantly fossil carbon source for marine gas hydrate, but also indicate that structural gas hydrate from at least certain cold seeps contains a component of methane produced during decomposition of non-fossil organic matter in near-surface sediment.This work was supported by the Office of Naval Research and Naval Research
Laboratory (NRL). Partial support was also provided by
the USGS Mendenhall Postdoctoral Research Fellowship Program to JWP, and NSF
Chemical Oceanography (OCE-0327423) and Integrated Carbon Cycle Research (EAR-
0403949) program support to JEB
The role of basalt replenishment in the generation of basaltic andesites of the ongoing activity at Arenal volcano, Costa Rica: evidence from clinopyroxene and spinel
The bulk composition of magma erupted from Volcan Arenal has remained
nearly constant (SiO2 = 53.6-54.9 wt%; MgO = 5.0-4.5 wt%) during
almost 30 years of continuous activity (1969-1996). None the less,
clinopyroxene (cpx) phenocrysts and their spinel inclusions record a
much more complex open-system evolution in which steady-state production
of the erupted basaltic andesitic magma is linked to episodic injections
of basalt into Arenal's magma conduit/reservoir system. High-resolution
major element zoning profiles (electron microprobe) on a large number of
phenocrysts (>14,000 analyses), tied to back-scattered electron (BSE)
images, have been used to assess the compositional characteristics of
the magmatic end members as well as the timing and dynamics of magma
replenishment events. No two cpx phenocrysts have exactly the same
zoning profile. The vast majority of our analyses record the
crystallization of cpx (Cr2O3 < 0.12 wt%; Mg# = 65-79; Al/Ti = 2-7)
from a liquid comparable to or more evolved than erupted magma
compositions. However, half of all cpx grains are cored by high-Cr cpx
(Cr2O3 = 0.2-0.72 wt%) or contain similar basaltic compositions as
abrupt growth bands in phenocrysts with and without high-Cr cores;
phenocrysts with high-Cr cpx occur throughout the ongoing activity. In a
few cases, high-Cr cpx occurs very near the outer margin of the grain
without an apparent growth hiatus, particularly in 1968/69 and 1992/93.
The main conclusions are: (1) all basaltic andesitic lavas erupted at
Arenal during the ongoing activity that began in July, 1968, are the
products of magma mixing, (2) clinopyroxenes record multiple
replenishment events of basaltic magma in contrast to the near constancy
of erupted bulk compositions, (3) some phenocrysts preserve records of
multiple interactions with basaltic magmas requiring magmatic processes
to operate on time-scales shorter than residence times of some
phenocrysts, (4) multiple occurrences of clinopyroxene with high-Cr rims
suggest that basalt replenishment events have occurred with sub-decadal
frequency and may predate eruption by months or less. From this we infer
that Arenal volcano is underlain by a continuously active, small-volume
magmatic reservoir maintained in quasi-steady state by basalt recharge
over several decades. The monotony of erupting Arenal magmas implies
that fractionation, recharge, ascent, and eruption are well balanced in
order for magmas to be essentially uniform while containing phenocrysts
with vastly different growth histories at the time of eruption
Warmed‐Over Flavor Development in Beef Patties Prepared by Three Dry Heat Methods
THE THIOBARBITURIC ACID (TBA) test and sensory evaluations were used to assess the effectiveness of three dry heat preparations—microwave, microwave/convection combination and oven broiling—in retarding warmed‐over flavor (WOF) development in ground beef patties. Animals finished on pasture, 60 and 120‐day grain were used. The oven broiling method was found to delay WOF development in refrigerated patties. Taste panelists failed to detect differences in flavor as a result of cooking or animal diet. There was a significant (P \u3c 0.001) negative correlation (r=−0.66) between aroma and flavor scores and TBA values over time. Panelists were able to detect deteriorating changes in flavor and aroma after refrigerated storage. Copyright © 1986, Wiley Blackwell. All rights reserve