Revisiting the dissolution of biogenic Si in marine sediments: a key term in the ocean Si budget
Acta Geochim (
Revisiting the dissolution of biogenic Si in marine sediments: a key term in the ocean Si budget
Patrick Frings 0 1 2 3 4
Diatoms 0 1 2 3 4
0 & Patrick Frings
1 11th International Symposium on Geochemistry of the Earth's Surface
2 Ocean Si cycle
3 Earth Surface Geochemistry, GFZ German Research Centre for Geosciences , 14473 Telegrafenberg, Potsdam , Germany
4 Department of Geoscience, Swedish Museum of Natural History , 10405 Stockholm , Sweden
Of the *240 9 1012 mol year-1 of biogenic silica (bSi) produced by diatoms and other silicifying organisms, only roughly 3%-4% escapes dissolution to be permanently buried. At the global scale, how, where and why bSi is preserved in sediment is not well understood. To help address this, I compile 6245 porewater dissolved Si concentrations from 453 sediment cores, to derive the concentration gradient at the sediment-water interface and thus diffusive fluxes out of the sediment. These range from \0.002 to 3.4 mol m-2 year-1, and are independent of temperature, depth and latitude. When classified by sediment lithology, predominantly siliceous sediments unsurprisingly have higher mean diffusive fluxes than predominantly calcareous or clay-rich sediment. Combined with the areal extent of these lithologies, the 'best-guess' global sedimentary bSi recycling flux is 69 9 1012 mol year-1.
Biogenic silica; Dissolution
1 Introduction
Ocean dissolved Si concentrations (hereafter [Si]) span two
orders of magnitude, from\1 lmol L-1 in surface gyres to
almost 200 lmol L-1 in parts of the deep ocean. Yet even
these regions are highly undersaturated—amorphous Si
solubility is ca. 1800 lmol L-1 at 20 C
(Gunnarsson and
Arno´rsson 2000)
. The ubiquitously high degree of
undersaturation attests to the efficiency with which biosilicifying
organisms, in particular the diatoms, can take dissolved Si
and precipitate it in their biogenic silica (bSi) skeletons.
This high degree of undersaturation also means that much
of the bSi produced dissolves after the organism’s death.
Dissolution begins in the euphotic zone, and continues
throughout the water column and into the upper sediment,
until the bSi is exhausted (leading to large parts of the
ocean floor devoid of siliceous remains) or the bSi reaches
equilibrium with sediment porewaters and dissolution
ceases.
The canonical figure for annual bSi production is
240 Tmol
(Tre´guer and De La Rocha 2013)
, approximately
259 annual inputs of Si to the ocean
(Frings et al. 2016)
.
Various approaches consistently indicate a large fraction—
around 50%–60%—is remineralised in the upper 100 m.
This leaves around 100 Tmol year-1 bSi as export
production
(Tre´guer and De La Rocha 2013)
. The fate of this
bSi is less well understood, though ca. 10 Tmol year-1
must be preserved to balance the inputs at steady-state. In
particular, the relative fractions of dissolution occurring in
the water column and in the sediments are not well
constrained, but has implications for how we quantify the past
and present Si cycle. Diffusive fluxes of Si across the
sediment–water interface can be used to estimate bSi
dissolution in the sediment; previous global scale estimates
include 8–38 Tmol year-1
(Treguer et al. 1995)
,
25.5 Tmol year-1
(Laruelle et al. 2009)
and 33–159
Tmol year-1
(Tre´guer and De La Rocha 2013)
. Clearly,
there is scope to improve these estimates. Here, I attempt
this by using a compilation of porewater [Si] profiles from
marine sediments to calculate diffusive Si fluxes out of
sediment.
2 Methods
I compiled [450 published porewater [Si] records from the
upper 40 cm of marine sediment cores (Fig. 1), assuming
that the different methods of coring and sample collection
yield comparable results. Most studies kept cores at in-situ
temperatures to avoid chemical changes, though no attempt
was made to identify and account for those that did not.
Both theoretical and empirical studies have shown that
porewater [Si] profiles can be described as an exponential
function
(e.g. McManus et al. 1995; Fig. 1)
:
½Si z¼ Casym
Casym
C0 e bz
where Casym is an asymptotic concentration, C0 is the
overlying water concentration, b an exponential constant
(cm-1) and z the depth below sediment surface (cm). Values
of Casym and b were obtained for each profile by searching
for the combination that minimised the RMSE between
predicted and measured values. The 5% of poorest model
fits were discarded. Differentiation of Eq. (1) at z = 0
provides the gradient across the sediment–water interface:
d½Si
dz z¼0
¼ b Casym
C0
which can then be used in Fick’s first law to calculate the
flux J (mol m-2 year-1) across the sediment–water
interface, assuming steady-state porewater DSi:
ð1Þ
ð2Þ
ð3Þ
J ¼
/D d½Si
dz
where D is dissolved Si diffusion coefficient which takes a
value of ca. 0.03 m2 year-1 at 25 C
(Wollast and Garrels
1971)
, and is corrected for sediment tortuosity and
temperature after
Boudreau (1996)
. In-situ temperature was
extrac (...truncated)