『Abstract
The distinctly different, εNd(0) values
of the Atlantic, Indian, and Pacific Oceans requires that the
residence time of Nd in the ocean (i.e., τNd)
be on the order of, or less than, the ocean mixing time of 〜500-1500
yr. However, estimates of τNd, based on river
influxes, range from 4000 to 15,000 yr, thus exceeding the ocean
mixing time. In order to reconcile the oceanic Nd budget and lower
the residence time by roughly a factor of 10, an additional, as
yet unidentified, and hence “missing Nd flux” to the ocean is
necessary. Dissolution of materials deposited on continental margins
has previously been proposed as a source of the missing flux.
In this contribution, submarine groundwater discharge (SGD) is
examined as a possible source of the missing Nd flux. Neodymium
concentrations (n = 730) and εNd(0) values
(n = 58) for groundwaters were obtained from the literature in
order to establish representative groundwater values. Mean groundwater
Nd concentrations and εNd(0) values were
used along with recent estimates of the terrestrial (freshwater)
component of SGD (6％ of river discharge on a global basis) to
test whether groundwater discharge to the coastal oceans could
account for the missing flux. Employing mean Nd concentrations
of the compiled data base (i.e., 31.8 nmol/kg for all 730 analyses
and 11.3 nmol/kg for 141 groundwater samples from a coastal aquifer),
the global, terrestrial-derived SGD flux of Nd is estimated to
range between 2.9×107 and 8.1×107 mol/yr.
These estimates are of the same order of magnitude, and within
a factor of 2, of the missing Nd flux (i.e., 5.4×107
mol/yr). Applying the SGD Nd flux estimates, the global average
εNd(0) of SGD is predicted to be -9.1, which
is similar to our estimate for the missing Nd flux (-9.2), and
in agreement with the mean (±S.D.) εNd(0)
measured in groundwaters (i.e., εNd(0) =
-8.9±4.2). The similarities in the estimated SGD Nd flux and corresponding
εNd(0) values to the magnitude and isotope
composition of the missing Nd flux are compelling, and suggest
that discharge of groundwater to the oceans could account for
the missing Nd flux. Future investigations should focus on quantifying
the Nd concentrations and isotope compositions of groundwater
from coastal aquifers from a variety of coastal settings, as well
as the important geochemical reactions that effect Nd concentrations
in subterranean estuaries in order to better constrain contributions
of SGD to the oceanic Nd budget.
Keywords: Nd; isotopes; seawater geochemistry; submarine groundwater
discharge; rare earth elements』
1. Introduction
2. Background
2.1. Oceanic Nd enigma
2.2. Submarine groundwater discharge
3. Methods
3.1. Data acquisition and analytical techniques
3.2. Model calculations
4. Results and discussion
4.1. Groundwater Nd concentrations and εNd(0)
values
4.2. Groundwater Nd fluxes to the ocean
4.3. Implications for paleoceanography
5. conclusions
Acknowledgments
References