『Abstract
Silicon isotopes in dissolved silicic acid were measured in the
upper four kilometers between 4゜N and 3゜S latitude at 110゜W longitude
in the eastern Equatorial Pacific. Silicon isotopes became progressively
heavier with silicic acid depletion of surface water as expected
from biological fractionation. The value of ε estimated by applying
a steady-state isotope fractionation model to data from all stations
between 4゜N and 3゜S was -0.77±0.12‰ (std. err.). When the analysis
was restricted to those stations whose temperature and salinity
profiles indicated that they were directly influenced by upwelling
of the Equatorial Undercurrent (EUC), the resulting value of ε
was -1.08±0.27‰ (std. err.) similar to the value established in
culture studies (-1.1‰). When the non steady state Rayleigh model
was applied to the same restricted data set the resulting value
of ε was significantly more positive, -0.61±0.16‰ (std. err.).
To the extent that the equatorial system approximates a steady
state these results support a value of -1.1‰ for the fractionation
factor for isotopes of Si in the sea. Without the assumption of
steady state the value of ε can only be constrained to be between
-0.6 and -1.1‰. Silicic acid in Equatorial Pacific Deep Water
below 2000 m had a near constant δ30Si of +1.32±0.05‰.
That value is significantly more positive than obtained for North
Pacific Deep Water at similar depths at stations to the northwest
of our study area (0.9-1.0‰) and it is slightly less positive
than new measures of the δ30Si of silicic acid from
the silicic acid plume centered over the Cascadia basin in the
Northeast Pacific (Si(OH)4>180μM, δ30Si
= +1.46±0.12‰ (SD, n=4). We show that the data from the equator
and Cascadia basin fit a general trend of increasing δ30Si(OH)4 with increasing silicic acid concentration in
the deep sea, but that the isotope values from the Northeast Pacific
are anomalously light. The observed level of variation in the
silicon isotope composition of deep waters from this single ocean
basin is considerably larger than that predicted by current models
based on fractionation during opal formation with no isotope effect
during dissolution. Confirmation of such high variability in deep
water δ30Si(OH)4 within individual
ocean basins will require reassessment of the mechanisms controlling
the distribution of isotopes of silicon in the sea.』
1. Introduction
2. Material and methods
3. Results and discussion
3.1. Water masses
3.2. δ30Si distribution
3.2.1. Surface waters
3.2.2. Intermediate and deep waters
3.3. Estimate of the fractionation factor, ε
3.3.1. Steady state model
3.3.2. Rayleigh model
3.3.3. Mixing effects on field estimates of ε
4. Conclusions
Acknowledgments
References