Scientific Reports
Scientific Reports
>Received: by revelle.UCSD.EDU (SMI-8.6/UCSDUUCPGENERIC.4)
id XAA09009 to ; Mon, 2 Nov 1998 23:05:37 GMT
Date: Mon, 2 Nov 1998 23:05:37 GMT
From: revelle!jacobson@sdsioa.UCSD.EDU (Dan Jacobson)
To: shipsked@ucsd.edu
Subject: ALARCON chief scientist report
Cc: jacobson@sdsioa.UCSD.EDU, master@sdsioa.UCSD.EDU
WEEKLY REPORT, ALARCON LEG 02, R/V REVELLE
R/V Revelle departed Pichilingue at 1118 local with SIO, UCSC,
Lawrence - Livermore (LLNL) and CICESE scientists. We sailed SE though
Cerralvo Channel to Alarcon Seamount. Using the exquisite bathymetric
chart and geologic interpretation made by Lonsdale on Leg 1, we began
our multidisciplinary survey which included rock dredging, heat flow
measurements, sediment coring and seismic profiling. Castillo,
Hilton, Forsythe and Hawkins made the first two rock dredges on inner
walls of the well-developed summit calderas of Alarcon Seamount. The
collection included lapilli tuffs with spindle shaped glass ejecta as
well as massive flow units of probable trachyandesite and alkalic
basalt. Mn-crusted partially dissolved "oyster" shells and small tube
worms suggest that the calderas had hydrothermal vent communities. The
axial ridge was dredged at six sites on "zero" age crust yielding
fresh pillows and sheet flow material. Axial ridge petrology varied
from plagioclase rich vitrophyres to aphyric basalt. One site dredged
from an abyssal hill scarp on crust about 0.65 My old gave altered
aphyric basalt heavily encrusted with Mn oxides. Fisher, Giambolvo and
Sclater made a heat flow survey on the southeast flank of the axial
ridge on 0.30 to 0.65 Ma crust. Measured conductive heat flow of 134
to 401 mW/m2 is 15 to 55% of that expected for lithosphere of that
age. The highest heat flow was found near the center of the sedimented
basin, close to acoustic washouts over basement highs seen on the 3.5
khz and seismic records. Lowest values are near the edges of the basin
adjacent to scarps of abyssal hills. A second HF survey was run
orthogonal to the first. Values ranged from 249 to 429 mW/m2 and
confirmed the association of heat flow with seafloor features. Kastner
and Ransom took a series of sediment cores using the multicoring (MC)
device and 3m long gravity cores along the heat flow survey tracks to
complement heat flow data. Pore water and sediment chemistry will be
used to test the hypothesis that the high HF may indicate fluid upflow
and the low HF values are at zones of seawater recharge. The water
chemistry data will also be used for calculating rates of fluid
flow. The MC cores were subsampled at high resolution at in-situ
temperature and in anaerobic environment for shore based studies. Pore
fluids were separated on board ship by centrifugation and analyzed for
alkalinity, pH, and ammonium. MC cores were subsampled for porosity
and the gravity cores were also sampled for rare gases and
permeability. Bottom water samples were collected for studies of
oxygen, rare gases, salinity and general chemistry. Herguera (CICESE)
subsampled MC cores and took a long piston core sample on the basin
margin for paleontologic studies of seawater temperature
variations. Volpe and Esser (LLNL) made continuous study of surface
seawater chemistry while underway. The intense sampling activity was a
complete success thanks to the expertise and round-the-clock work of
Resident Techs Bob Wilson and Seth Mogk. We all owe them a vote of
appreciation and thanks for their help. Chris Massell gave us
continual updated versions of the bathymetry and made detailed charts
of areas we sampled. SIO and CICESE students and post-docs, and our
volunteers were enthusastic and able assistants in all aspects of the
work. The officers and crew of Revelle also gave us expert
assistance. We give our thanks to them and Capt. Chis Curl for their
excellent seamanship and a job well done. Needless to say - we ate
very well. Thanks Paul and Bob.
J. Hawkins, Chief Scientist
This cruise is being funded by a grant from the University of California.
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