Sensitivity of silicon isotopes to whole-ocean changes in the silica cycle

Marine Geology 217 (2005) 267 – 282 www.elsevier.com/locate/margeo

Sensitivity of silicon isotopes to whole-ocean changes in the silica cycle Christina L. De La RochaT,1, Michael J. Bickle2 Department of Earth Sciences, University of Cambridge, Downing St, Cambridge CB2 3EQ, UK Received 13 July 2004; received in revised form 1 November 2004; accepted 2 March 2005

Abstract A 2-box model has been used to assess the impact of both long- and short-term budgetary imbalance in the silica cycle on the average silicon isotopic composition (y30Si) of the ocean and marine sediments. Over a 100-ky time span, such as a Quaternary glacial cycle, a sustained change in the riverine flux of silicon to the oceans could alter the average y30Si of seawater and the average y30Si of opal outputs by a few hundredths to a few tenths of permil. This would be largely tied to a change in the y30Si of silicon entering the ocean due to a shift in the proportion of riverine and non-riverine sources of silicon. A doubling of the riverine flux of silicon would have little impact on average marine y30Si, but a sustained halving of river inputs could interfere with use of y30Si as a tracer of nutrient utilization. Studies on the longer term focussed on the transition from a high silicic acid to low silicic acid ocean associated with the rise of the diatoms. This transition is marked by drop in the average y30Si of seawater from greater than + 1.9x down to about + 0.8x. The isotopic composition of diatom opal, however, has an isotopic composition that sticks close to the + 0.8x of the inputs and is thus unlikely to provide information about the transition to the low silicic acid ocean of the modern day. However, the y30Si of opal produced in the deep sea (for example, by sponges) should document this transition. D 2005 Elsevier B.V. All rights reserved. Keywords: silicon; y30Si; long-term silica cycle; 2-box model; diatom; silicon isotopes

1. Introduction T Corresponding author. Tel.: +44 1223 333 479; fax: +44 1223 333 450. E-mail addresses: [email protected], [email protected] (C.L. De La Rocha), [email protected] (M.J. Bickle). 1 Present address: Alfred-Wegner-Institut fqr Polar und Meeresforschung, Columbusstrage, 27568 Bremerhaven, Germany. Tel.: +49 471 4831 1040; fax: +49 471 4831 1149. 2 Tel.: +44 1223 333 484; fax: +44 1223 333 450. 0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2004.11.016

The silicon isotopic composition of biogenic opal is being increasingly used to document in the silica cycle in the modern day and back through time (De La Rocha et al., 1997, 1998, 2000; Brzezinski et al., 2002; De La Rocha, 2003; Wischmeyer et al., 2003; Ding et al., 2004; Varela et al., 2004). It has, for example, been used to look at silicic acid utilization by diatoms on a

C.L. De La Rocha, M.J. Bickle / Marine Geology 217 (2005) 267–282

regional scale over the glacial–interglacial cycles of the Quaternary (De La Rocha et al., 1998; Brzezinski et al., 2002). Silicon isotopes are also of interest over longer time spans and over ocean-wide scales, with a low-resolution record of sponge spicules from across the Eocene–Oligocene Boundary showing large variations and potentially indicating instability in the marine silica cycle (De La Rocha, 2003). Important to both of these applications is the question of the extent to which whole-ocean changes in silicon isotopic composition are possible and recorded in the sedimentary record. If the variations are sizeable and retained in sedimentary opal then they provide a way for us to identify times of imbalance between Si outputs and inputs, and of change in oceanic concentrations of silicic acid. However, this would also mean that the use of silicon isotopes as a local proxy for nutrient utilization over glacial– interglacial time scales could be compromised by the shifting isotopic composition of the entire ocean. Silicon has three stable isotopes, 28Si, 29Si, and 30 Si. Slight variations in the abundances of these isotopes are generally reported in permil (x) as y30Si:

6 5 4

δ30Si (‰)

268

3 2 1 0 -1 1.0

0.8 0.6 0.4 0.2 Fraction of silicic acid remaining

0.0

ð1Þ

Fig. 1. Rayleigh distillation of silicon isotopes in the surface ocean. The lines track the y30Si of opal and silicic acid during the draw down of silicic acid from an initial y30Si of silicic acid of +0.8x and a fractionation factor (a) for opal biomineralization of 0.9989. As the fraction of silicic acid remaining in solution ( f) decreases, the y30Si of silicic acid (the solid line) and of the total amount of opal that has been produced since f = 1 (small dashed line) increases. This opal represents the opal that accumulates on the seafloor and can never have a y30Si value greater than that of the initial silicic acid. The long dashed line represents the y30Si of opal produced at each individual moment in time during the draw down, and when a = 0.9989 is always 1.1x lower than the y30Si of the silicic acid.

where R sam and R std are the 30Si/28Si ratio in a sample and standard (NBS28), respectively. Silicic acid in modern seawater generally falls into the y30Si range of +0.4x to +1.0x, largely reflecting the y30Si of the inputs of Si to the ocean (De La Rocha et al., 2000). During biomineralization, diatoms discriminate against the heavier isotopes of silicon, producing opal with a y30Si value that is 1.1x to 1.5x relative to the silicic acid used for its construction (De La Rocha et al., 1997, 2000; Milligan et al., 2004; Varela et al., 2004). Because of this discrimination, the y30Si of silicic acid in surface seawaters of the modern ocean is higher than that of deeper waters (De La Rocha et al., 1997, 2000; Wischmeyer et al., 2003), reaching values as high as +3x (Varela et al., 2004) in a process known as Rayleigh distillation (Fig. 1). Diatom opal collecting on the seafloor thus gives some indication of the degree of silicic acid utilization at the time of its production. Silicon isotopes have been used to infer that silicon utilization by diatoms in the Southern Ocean occurs to

a greater extent during interglacials than during glacials (De La Rocha et al., 1998; Brzezinski et al., 2002). It has long been wondered, however, how much the approximately 1x drop in diatom y30Si between interglacials and glacials is due, not to a regional shift in silicic acid utilization, but to a wholeocean change in the y30Si of silicic acid. A climatically linked change in the flux of silicon to the ocean via rivers could drive such a shift by changing the relative degree of silicic acid consumption (and therefore Rayleigh distillation) in surface waters. It could also drive a shift by changing the isotopic composition of the input flux of silicon to the sea. Large variations in the y30Si of sedimentary opal have also been seen over million year time scales at the Eocene–Oligocene boundary (De La Rocha, 2003). The cause of these variations is not yet known. One of the several ideas proposed was that these variations reflect times of severe imbalance in the silica cycle at a time when the cycle was settling down

d30 Si ¼

R sam  Rstd  103 Rstd

C.L. De La Rocha, M.J. Bickle / Marine Geology 217 (2005) 267–282

into the mode of operation we are familiar with in the modern day (Siever, 1991). At present, silicic acid concentrations in the ocean are relatively low and diatoms dominate the removal of silicon as opal, or amorphous, hydrated silica (Nelson et al., 1995; Tre´guer et al., 1995). In order to improve the use of Si isotopes as a paleo-proxy of silica cycling, we pose two simple questions. What impact does a change in the riverine flux of silicon to the ocean have on the average y30Si of silicic acid and diatom opal? How does the y30Si of silicic acid and diatom opal respond to a severe drop in average marine concentrations of silicic acid driven by the increased sedimentation of diatom opal? The silica cycle of the modern ocean appears to be at steady state, with inputs and outputs both somewhere in the neighborhood of 7  1012 mol of silicon per year (Tre´guer et al., 1995; Elderfield and Schultz, 1996; DeMaster, 2002). Rivers provide ~ 85% of the dissolved silicon that comes in to the ocean, and hydrothermal vents and the low-temperature weathering of aeolian dust and seafloor basalt provide the remainder of the external inputs (Tre´guer et al., 1995; Elderfield and Schultz, 1996). Outputs of silicon occur predominantly in the form of diatom opal buried on the sea floor (Tre´guer et al., 1995; Nelson et al., 1995; DeMaster, 2002), although radiolarian tests, sponge spicules, and silicoflagellate scales also contribute to this flux. The average concentration of silicic acid in the modern ocean is 70 AM, and values range from sub-AM up to over 250 AM. The lowest concentrations of silicic acid (generally b 10 AM) occur in the surface ocean where diatom opal is formed (Tre´guer et al., 1995). The input of silicon to the ocean via rivers during cold, dry glacial periods is probably not the same as it is during interglacials (Froelich et al., 1992). The amount of silicon delivered by rivers is controlled of two major processes, the chemical breakdown of continental rocks and the formation of clays (Stallard and Edmond, 1983; Murnane and Stallard, 1990; Froelich et al., 1992; Drever, 1997). Rates of rock weathering and clay formation are in turn tied to factors, such as rainfall, temperature, vegetation or ice coverage of the ground, the steepness of terrain, and the movement of glaciers, many of which vary over glacial–interglacial cycles.

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There is a large range in the estimates of riverine silicon fluxes during glacials. Glacial weathering fluxes of material in general could have been a mere 20% of glacial values (Davis et al., 2003). Si fluxes might be decoupled from this, however, by virtue of being more controlled by the lock up of Si into clays than by the leaching of Si from rocks (Murnane and Stallard, 1990). One interpretation of the marine Ge/Si record puts glacial river fluxes of silicon at 250% of modern, interglacial values (Froelich et al., 1992) although the recent discovery of an additional marine sink for Ge (Hammond et al., 2000; King et al., 2000) suggests that this is an overestimate. Although we may be a long ways off from sorting out exactly how Si inputs vary over glacial–interglacial cycles, if the estimates range from 0.2 to 2.5 times the interglacial flux, the continued use of silicon isotopes as a nutrient proxy requires an assessment of the impact of changing river fluxes on average marine y30Si. Over the Phanerozoic, the output of silicon has exceeded its input to the point where the amount of silicic acid in the ocean has declined by a factor of about 20 (Siever, 1991). The modern, close-to-balanced silica cycle with a surface ocean strongly depleted in silicic acid is a relatively recent development, by no means representative of the silica cycle further back in time (Harper and Knoll, 1975; Maliva et al., 1989; Siever, 1991; Racki and Cordey, 2000). For much of geologic history, silicic acid concentrations in the ocean have been much higher than they are at present. The evolutionary emergence and radiation of opal biomineralizing sponges and radiolarians in the Paleozoic and then of diatoms in the Mesozoic drew marine silicic acid concentrations down by 1000 AM (Siever, 1991) in what was likely a series of steps (Racki and Cordey, 2000). This severe decline is supported by changes in the distribution and fabric of cherts, and the degree of silicification of sponges and radiolarians in the fossil record (Harper and Knoll, 1975; Maliva et al., 1989; Siever, 1991; Maldonado et al., 1999; Racki and Cordey, 2000). But the details of the transition from a high-silicon, bpre-diatomQ ocean to a low-silicon, bdiatom dominatedQ ocean remain elusive. Just as there is a possibility that imbalances in the silica cycle over glacial–interglacial cycles of the Quaternary might generate a visible signal in sedimentary y30Si, large imbalances between inputs and

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outputs of silicon to the ocean over the Phanerozoic may also be visible in the sedimentary record. If so, it may be possible to pinpoint in time and quantify the decline in the silicic acid content of ocean waters due to the evolution and radiation of silica biomineralizing organisms in the ocean. To investigate the relationship between silica budgets and the silicon isotopic composition of marine sediments over time we have constructed a 2-box model of silica cycling in the oceans. This model takes in to account the input and output of silicon from the ocean, both in terms of amount and isotopic composition, relegates opal production to the euphotic zone (the upper box with a volume equal to that of only 3% of the total ocean), and is capable of being run at 1-yr resolution for tens of millions of years to assess the sensitivity of silicon isotopes as tracers of the silica cycle. The purpose of this simple model and the runs we have put it through is not to provide a picture of exactly how the silicon isotopic composition of the

Input

oceans and sediments has changed over geologic time. We aim merely to investigate the potential visibility of long-term imbalance between inputs and outputs of Si to the sea in the sedimentary record of y30Si.

2. Materials and methods 2.1. Description of the model Fig. 2 shows the basic set-up of the 2-box silicon isotope model. The ocean has been split into a deep box, and a surface box representing the sunlit upper 100 m of the ocean (Table 1). Silicon is taken up into and dissolved from biogenic opal, exchanged between surface and deep boxes, brought in through river and hydrothermal input, and removed as the sinking and sedimentation of biogenic opal. Initial concentrations of dissolved silicon (DSi) in the surface (C surf) and deep (C deep) boxes are assigned at To (Table 2) and evolve over the course of a model run.

FinRin Surface

Production Downwelling

Surface

BSiprodRBSi

Dissolution

WexCsurfRsurf

RBSiBSiprod/2

WexCdeepRdeep

Upwelling

DissdeepRBSi

Dissolution

A

Deep Sedimentation

B

Deep FoRBSi

Fig. 2. Basic set-up of the model, including both mass fluxes and isotopes. Dissolved Si with the isotopic composition, R in, is input ( F in) to the model in the surface box, and consists of both riverine and non-riverine fluxes. Dissolved Si is converted to opal (BSiprod) in surface waters, where half of it dissolves (BSiprod/2). The remaining opal sinks to the deep ocean, where most of it (Dissdeep) dissolves and a small fraction ( F o) is exported from the ocean into the sediments. The degree to which the ocean is out of steady state with respect to the overall Si content is expressed by the ratio of F o to F riv. Water is exchanged between surface and deep boxes via upwelling and downwelling (Wex).

C.L. De La Rocha, M.J. Bickle / Marine Geology 217 (2005) 267–282 Table 1 Constants Constant

Value

Units

Description

F in R std

7  1012 0.0335320

mol yr1

R in a V surf

0.0335589 0.9989 3.6  1019

L

V deep Wex

1.334  1021 1.37  1018

L L yr1

P eff

0.035

input of Si to ocean 30 Si/28Si of standard (NBS-28) 30 Si/28Si of input Si fractionation factor volume of upper 100 m of ocean ocean volume below 100 m volume upwelled (downwelled) each year opal preservation efficiency (fraction of produced BSi exported to sediments)

Water is exchanged between the surface and deep boxes through upwelling and downwelling of a volume of water each year (Wex) set for a realistic deep ocean mixing time of 1000 yr (Broecker and Peng, 1982). The flux of silicon in to the ocean ( F in) comes into the surface box and includes both riverine and non-riverine components. Silicon is removed by the sedimentation of biogenic opal. Opal (BSiprod) is produced in the surface box. Half of the produced opal dissolves straight back into the surface box, as occurs in the modern ocean (Nelson et al., 1995). The change in the concentration of DSi in the surface box over time is thus: Vsurf

BSiprod dCsurf ¼ Fin   Wex Csurf þ Wex Cdeep : dt 2 ð2Þ

The half of the biogenic opal that does not dissolve in the surface box (BSiprod/2) is exported to the deep. The portion of this sinking opal that dissolves in the deep (Dissdeep) is controlled by the preservation efficiency ( P eff; Table 1). Changes in the concentration of DSi in the deep box include this input from dissolution as well as input from the downwelling of surface water and the output of DSi via upwelling of deep water into the surface: Vdeep

dCdeep ¼ Wex Csurf  Wex Cdeep þ Dissdeep : dt

ð3Þ

By this set-up, the total flux of silicon out of the ocean is the amount of opal exported from the surface box minus what dissolves in the deep, or BSiprod / 2 Dissdeep.

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In terms of setting the balance between silicon inputs and outputs, the model may be run in either of two ways. The input of silicon may be fixed and the production (and thus export) of silica varied to provide a budget that is out of steady state to a chosen degree. Alternatively, silica production may be held constant and the input flux varied. In either event, the preservation efficiency of opal (i.e. the burial flux of opal ( F o) divided by opal production (BSiprod), denoted P eff) has been set at a constant value of 3.5% (Tre´guer et al., 1995; DeMaster, 2002). Once the river flux and amount of productivity at each time step are established, the change in the isotopic composition of DSi in the surface box (R surf) is just the sum of the input fractions times their isotopic ratios (R in, R BSi, and R deep as defined in Tables 1 and 2) minus the output fraction multiplied by its 30Si/28Si ratio (R surf): Vsurf Csurf

BSiprod dRsurf ¼ Fin Rin  RBSi dt 2  Wex Csurf Rsurf þ Wex Cdeep Rdeep : ð4Þ

Likewise changes in the isotopic composition of DSi in the deep box represent the balance between inputs and outputs multiplied by their 30Si/28Si ratios: Vdeep Cdeep

dRdeep ¼ Dissdeep RBSi þ Wex Csurf Rsurf dt  Wex Cdeep Rdeep :

ð5Þ

Opal reaching the sediments is largely produced by blooms of diatoms that rapidly deplete the silicic acid concentration of surface waters. Because of this the isotopic composition of opal is not simply the y30Si of silicic acid minus the 1.1x fractionation associated with diatom biomineralization of opal, but rather evolves over the course of silicic acid utilization because of Rayleigh distillation (Fig. 1). Thus, in the model, the isotopic composition of opal produced in Table 2 Variables Variable

Units

Description

C surf C deep R surf R deep

AM AM

surface [DSi] deep [DSi] 30 Si/28Si of surface ocean 30 Si/28Si of deep ocean

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surface waters is set, following the Rayleigh distillation model, by R surf, a (Table 1), the fractionation factor associated with opal biomineralization, and f, the fraction of surface DSi that remains unutilized at the end of the accumulation of BSiprod: RBSi ¼ RDSi

ð1  f a Þ : ð1  f Þ

ð6Þ

y30Si values for DSi and BSi (Table 3) are then calculated from the ratios using Eq. (1). Models more complex and sophisticated than this one exist for silicon cycling (e.g. Nelson et al., 1995; Gnanadesikan, 1998; Heinze et al., 1998) and even to track the distribution of silicon isotopes in the ocean (Wischmeyer et al., 2003). The detail in these models, however, goes far beyond what is needed in order to assess changes in the whole-ocean average y30Si of surface and deep water silicic acid and diatom opal. The detailed circulation of water in these models, tuned to the modern ocean, also may not be useful for runs reaching far back in geologic time. The advantage of the simple 2-box model presented here is that it can be easily run over millions of years at 1yr resolution. 2.2. Initialization of model runs All runs were begun from steady state starting points. For the Quaternary runs, the model ocean was filled with 70 AM silicic acid with a y30Si equal to the y30Si of the inputs. This input y30Si was set at +0.8x and represents a mass balance of 85% of riverine silicic acid (Tre´guer et al., 1995; Elderfield and Schultz, 1996) with a y30Si of +1.0x (De La

Table 3 Calculated variables Variable

Definition

f

fraction of surface DSi remaining after BSiprod 30 Si/28Si of BSi amount of BSi dissolved in deep ocean during sinking to seafloor y30Si of surface DSi y30Si of deep DSi y30Si of BSi

R BSi Dissdeep

y30Sisurf y30Sideep y30SiBSi

Units Equation =1  BSiprod / C surfV surf

mol yr1

x x x

=R DSi(1  f a ) / (1  f) =BSiprod / 2  R o/iP eff

=(R surf / R std  1)  103 =(R deep / R std  1)  103 =(R BSi / R std  1)  103

Rocha et al., 2000) and 15% silicon from nonriverine sources (Tre´guer et al., 1995; Elderfield and Schultz, 1996) with a y30Si of 0.3x (Douthitt, 1982; Ding et al., 1996; De La Rocha et al., 2000). The number of moles of silicon in and out of the model was set to the modern day value of 7  1012 (Tre´guer et al., 1995; Elderfield and Schultz, 1996; DeMaster, 2002), and the amount of biogenic silica production was set accordingly for an opal preservation efficiency of 3.5% (Tre´guer et al., 1995; DeMaster, 2002). The model was initialized by being run under these conditions until the y30Si of silicic acid and opal no longer changed. The same procedure was followed to attain the steady state starting point for the high silicic acid bpre-diatomQ ocean, except that the ocean was initially filled with 1000AM silicic acid. The Quaternary ocean settled at a silicic acid concentration of 6.4 AM in the surface and 72 AM in the deep. The steady state y30Si values were +1.1x for silicic acid in the surface box, and +0.8x for silicic acid in the deep. Opal y30Si equalled the value of +0.8x of the inputs, reflecting the nearly complete degree of draw down of surface silicic acid. In the bpre-diatomQ ocean, concentrations of silicic acid settled at 936 AM in the surface and 1002 AM in the deep. The y30Si of silicic acid settled at +1.8x in the deep ocean and +1.9x in the surface. These relatively high y30Si values are not due to Rayleigh distillation of the surface box at this time when opal production represents a tiny fraction of the silicic acid available in the surface. It is the complete lack of Rayleigh distillation in the bpre-diatomQ ocean that results in these high values. Because the fractionation factor (a = 0.9989) requires that opal produced has a y30Si value 1.1x lower than that of silicic acid in the absence of any Rayleigh distillation, the output of opal with a steady state composition equal to the +0.8x of the inputs requires ocean waters 1.1x higher than +0.8 (i.e. about +1.9x). It is likely that surface water values in the bpre-diatomQ ocean were even higher than this, as the fractionation of silicon isotopes by sponges, at about 3.8x, is greater than it is by diatoms (De La Rocha, 2003). Because there is nothing currently known about fractionation by radiolarians, the other major component of opal sediments in the bpre-diatomQ ocean, we have stuck

C.L. De La Rocha, M.J. Bickle / Marine Geology 217 (2005) 267–282

to the use of the known fractionation of 1.1 by diatoms. 2.3. Model runs The 2-box silicon isotope model was run to ask two questions, one for the Quaternary ocean and one for the bpre-diatomQ ocean. The Quaternary runs focussed on the impact of a change in the riverine flux of silicon to the oceans on the y30Si of diatoms and silicic acid. The bpre-diatomQ ocean runs considered the impact of a large and sustained excess export of silicon as opal from the ocean. In the Quaternary runs, river fluxes of silicon were either diminished (by 10%, 50%, and 80%) or increased (by a factor of 2.5). Since the non-riverine portion of the input was held constant, these changes resulted in a shift in the y30Si of the input from +0.8x to +0.79, +0.66, +0.39, and +0.82x, respectively. BSiprod was set to 99% of the total amount of silicic acid present in the surface box at the beginning of each model year. The evolution of the y30Si of silicic acid in the surface and deep and of diatom opal was then tracked for 100 ky. For the bpre-diatomQ ocean runs, the flux of silicon into the ocean was held constant at the modern value of 7  1012 mol/yr (Tre´guer et al., 1995). The gross amount of biogenic silica production was allowed to vary so that the output of silicon from the ocean exceeded its input, but the preservation of opal in the sediments always equalled 3.5% of BSiprod. Holding preservation efficiency constant is somewhat of an oversimplification, as silica dissolution in seawater is tied to silicic acid concentrations (Hurd, 1972). Given the lack of Rayleigh distillation of silicon isotopes in surface waters at high silicic acid concentrations, higher preservation efficiencies at these times (i.e. lower levels of opal production for a given output of Si from the system) are unlikely to have resulted in a different y30Si values for opal produced in surface waters than tabulated by the model runs at the modern value of 3.5%. In one set of the bpre-diatomQ ocean runs, the ratio of inputs to outputs was held constant at 1.05 or 1.50, depending on the run. In a second set of runs, BSiprod increased exponentially over time, mimicking an evolutionary radiation of organisms from a small initial population. In the second set of runs, the ratio

273

of outputs to inputs was initially set at 1 and then allowed to increase, via an increase in opal production, as:
ð7Þ BSiprod ¼ 2:0  1014 elt with the rate constant (l) set at 3  10 8 , or 180  108 yr1.

3. Results 3.1. Reproduction of modern conditions When run into steady state from initial conditions of 70 AM silicic acid with a y30Si of +0.8x and an average fractionation factor (a) of 0.9989, the 2-box silicon isotope model reasonably reproduced modern ocean conditions. The surface and deep silicic acid concentrations of 6.4 and 72 AM obtained resemble modern values (e.g. Tre´guer et al., 1995). Silicic acid in the surface and deep settled at +0.85x and +0.80x, respectively, and the y30Si of the opal formed took on the value of that of the inputs. The steady state y30Si values all fall within the estimates made for silicic acid and opal in the modern ocean (De La Rocha et al., 2000; Wischmeyer et al., 2003; Varela et al., 2004). This steady state deepwater value of +0.8x is within the average value of +1.1 F 0.3 (n = 69) for all waters deeper than 200 m measured so far (De La Rocha et al., 2000). Likewise the estimate of +0.8x for the buried opal essentially falls within the range of +0.9 to +1.9x estimated from a handful of samples for opal outputs from the modern ocean (De La Rocha et al., 2000). The global average surface water y30Si value of 0.85x is only slightly elevated from the deepwater value despite the near complete drawdown (and thus extensive Rayleigh fractionation) of silicic acid in the surface. This is due to the dissolution of half of the opal, with its y30Si of +0.8x, in the surface box. Although surface waters in certain areas at times of intense silica production tend to have y30Si values higher than this (e.g. +1 to +3x; De La Rocha et al., 2000; Varela et al., 2004), for a global value integrated over an entire year (not just the growing season), it is a reasonable value.

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3.2. Silicon isotope variations for Quaternary changes in river fluxes Fig. 3 shows the impact on silicic acid concentrations, opal production, and y30Si of both an abrupt and sustained change in the riverine silicon input flux to the ocean of 0.5 and 2.5. In the case of the 2.5fold increase, global average diatom opal y30Si almost immediately took on the new average y30Si of the inputs, shifting from the steady state Quaternary ocean value of +0.8x to +0.82x (Fig. 3B). In the case of the halving of the riverine silicon flux, average marine diatom opal took about 40 ky to reach the new average input value of y30Si of +0.66x. Other runs, at 0.2-, 0.9-, and 5-times the modern river silicon flux yielded final values for opal y30Si of

[Si(OH)4] (µM)

180 150

+0.39x, +0.79x, and +0.86x, respectively. These values all match the calculated y30Si of the inputs, assuming riverine y30Si = +1.0x and non-riverine y30Si = 0.3x. Cumulatively, these results suggest that changes in the input of silicon to the ocean over glacial– interglacial time scales will only shift the y30Si of average marine diatom opal as a result of a shift in the y30Si of the inputs. Other factors have little or no impact (Table 4). In the case of the halved river flux, a doubling of the preservation efficiency from 3.5% to 7%, for example, still results in opal of +0.66x, despite higher equilibrium concentrations of silicic acid in surface and deep waters (Table 4). The degree of Rayleigh distillation in the surface box on the isotopic composition of opal outputs is also of

A

D

B

E

120 90 60 30 0 0.9

δ30Si (‰)

0.8 0.7 0.6 0.5 0.4

Production (Tmol Si y-1)

0.3 500

F

C

400 300 200 100 0 0

20

40

60

Time (kyr)

80

100 0

20

40

60

80

100

Time (kyr)

Fig. 3. Impact of diminished river inputs to the ocean on y30Si. Panels A–C show the response to a 2.5-fold increase in river input of Si(OH)4 to the ocean by (A) surface (dashed line) and deep (solid line) silicic acid concentrations, (B) y30Si of surface Si(OH)4 (long dashed line), deep Si(OH)4 (solid line) and diatom opal (small dashed line), and (C) opal production. Panels D–F reflect the same parameters but in the case of a halving of river inputs of silicon to the ocean.

C.L. De La Rocha, M.J. Bickle / Marine Geology 217 (2005) 267–282 Table 4 The impact of altering preservation efficiency and percent draw down Case for 0.5 river flux

f = 0.99, P eff = 3.5% f = 0.80, P eff = 3.5% f = 0.99, P eff = 7.0%

y30Si opal

Silicic acid at 100 ky

20 ky (x)

50 ky (x)

100 ky (x)

Surface

Deep

+0.72

+0.67

+0.66

+0.71

+0.67

+0.66

+0.69

+0.66

+0.66

3.2 AM +0.71x 4.0 AM +1.10x 1.6 AM +0.71x

40.8 AM +0.67x 41.6 AM +0.70x 19.0 AM +0.67x

negligible impact to average opal y30Si. For example, if opal production in the halved river input scenario were held at an 80% drawdown of silicon in the

[Si(OH)4] (µM)

surface box instead of the 99% of Fig. 3D–F, the y30Si of the outputs would still fall within 0.01x of that of the outputs in Fig. 3E (Table 4). The only significant ocean-wide impact on y30Si that a sustained fractional removal of 80% of the surface silicic acid would have would be to increase the average y30Si of silicic acid in surface waters by 0.3x (Table 4). 3.3. Silicon isotope variations tied to significant drops in the silicon content of the bpre-diatomQ ocean The y30Si of diatom opal showed very little response to a linear draw down of silicic acid from 1000 AM to 0 AM, despite the more sizeable response in the y30Si of both surface and deep silicic acid (Fig. 4). When outputs exceeded inputs by a constant 5%,

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F

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Fig. 4. Results of the longer term model runs with a fixed imbalance between silicon inputs and outputs. Panels A–C show the drop in silicic acid concentrations and changes in y30Si when the ratio of outputs to inputs of silicon to the ocean is held steady at 1.05. Panels D–F reflect an output to input ratio of 1.50. Symbols are as in Fig. 3.

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there was virtually no response in the isotopic composition of diatom opal (Fig. 4B). When outputs exceeded inputs by 50% each year and marine silicic acid concentrations were drawn down to zero in less than a half a million years (Fig. 4D– F), diatom opal y30Si shifted in value by 0.6x, large enough to be seen in the sedimentary record. Although the imbalance between inputs and outputs had to be large to show up as a shift in diatom opal y30Si, the shift in the y30Si of deepwater silicic acid was sizeable in both the moderate and extreme case. In both scenarios, deepwater y30Si variations were on the order of 0.8x and were spread out over a broad range of the draw down (Fig. 4B,E). By comparison, surface water silicic acid y30Si had

[Si(OH)4] (µM)

1000

most of its variation relegated to the last few years of the model runs. This was the time when silicic acid concentrations approached 0 and marine silicic acid moved towards the y30Si of the input flux (Fig. 4B,E). When the rate of draw down of silicic acid was accelerated over time due to the exponential increase in the amount of opal produced each year (Fig. 5), the change in the y30Si of diatom opal was slightly greater than in the case of the fixed rate of draw down. In the scenario depleting the ocean of silicic acid in less than half a million years (Fig. 5D–F), the y30Si of diatom opal increased by 1.0x, with half of the isotopic shift contained within the final 20% of the time.

A

D

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Fig. 5. Impact of exponentially increasing opal production on silicic acid concentrations and y30Si from an initial output to input ratio of 1.0. Panels A–C detail silicic acid concentrations and y30Si as opal production (and thus the ratio of outputs to inputs of silicon) increases exponentially over time (t) as BSiprod = (2.0  1014)e3E  8t . In panels D–F, the increase in opal over time is BSiprod = (2.0  1014)e180E  8t . Symbols are as in Fig. 3.

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4. Discussion 4.1. Little impact of changing river flux of silicic acid on d30Si records over glacial–interglacial cycles in the Quaternary Runs of the 2-box model suggest that a decrease in the flux of riverine silicon to the oceans would have to be quite extreme to significantly contribute to the observed decrease in y30Si between interglacial and glacial periods. In the model, a 10% decline in river fluxes had virtually no impact on the average y30Si of sedimenting opal, shifting it by only 0.01x. Even a 2-fold or 5-fold increase in the river flux shifted the average sedimentary y30Si by +0.02x to +0.04x. It was only extreme decrease in river fluxes that caused changes in the average y30Si of opal large enough to interfere with the translation of the signal in the sediment cores into changes in nutrient utilization over time. The 0.14x and 0.41x shifts associated with a 50% and an 80% decrease in river fluxes are large next to a glacial–interglacial change totalling up to 1.0x (De La Rocha et al., 1998; Brzezinski et al., 2002). Interestingly, it was not the draw down of silicic acid concentrations in the surface ocean and the associated Rayleigh distillation of isotopes that result in the shifts in opal y30Si as riverine fluxes of silicon diminished. If this had been the case, the y30Si of average seawater and diatom opal would have risen exponentially (as in Fig. 1) as river inputs fell. Instead, y30Si decreased (Fig. 3E) as silicic acid concentrations decrease (Fig. 3D). The factor that drove the modelled changes in silicic acid and opal y30Si was the average y30Si of the silicon input flux. The y30Si of +0.8x assigned to the silicon inputs to the ocean in this model at steady state represented a mixture of bigneousQ silicon at 0.3x, and briverineQ silicon at +1.0x. When the riverine portion of that input decreased, the input y30Si moved closer to the igneous value. And when the river flux increased, the input y30Si moved closer to the river value. Because riverine silicon made up 85% of the inputs in the steady state case, the 2.5-fold and 5-fold increase in river fluxes would not shift the y30Si of the inputs by nearly as much as a 2.5-fold or 5-fold decrease would. Thus any decrease in river fluxes relative to inputs from hydrothermal vents, seafloor

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basalt dissolution, and dust dissolution has a much more marked impact on average marine y30Si than any increase in river fluxes. 4.2. Assessing the glacial–interglacial variability in global riverine d30Si All of the above results presume, of course, that the average y30Si of riverine silicic acid does not vary over time. At present, one likely candidate for the control of the y30Si of freshwater is the formation of clays during the weathering of rocks (De La Rocha et al., 2000). If this is true and if the fraction of weathered sequestered into clays significantly differs between glacials and interglacials, as implied by the Ge/Si data (Froelich et al., 1992), then the assumption of a constant riverine y30Si is false. If we accept the inverse of the silicic acid concentration ([silicic acid]1) as a monitor of weathering intensity, and that the fraction of weathered Si sequestered into clays increases as [silicic acid]1 decreases (Murnane and Stallard, 1990; Froelich et al., 1992) then we have a means of assessing the potential change in average riverine y30Si over glacial–interglacial cycles. This means is quite limited at the moment because there exists very little data on the y30Si of rivers (De La Rocha et al., 2000; Ding et al., 2004) with which to test how riverine y30Si varies with weathering intensity. Far and away the most complete data set on riverine y30Si comes from the Yangtze (Ding et al., 2004). In the upper reaches of the river, there is a severe relationship between y30Si and weathering intensity (y3 0 Si = 1090.9 / [silicic acid]  8.1; r 2 = 0.72; n = 9). In the middle and lower reaches of the river, however, the relationship is muted and far from significant (y30Si = 350.6 / [silicic acid]  1.1; r 2 = 0.08; n = 10). There is no way the relationship observed in the upper reaches can be representative of rivers as a whole, as it predicts a y30Si of 1.9x for the global average [silicic acid]1 of 1/177 (i.e. 0.0057; Meybeck, 1988). The relationship from the middle and lower reaches of the river is more reasonable, predicting a global average river y30Si of +0.9x, close to the average deep water value of seawater (De La Rocha et al., 2000). Both slopes present an interesting puzzle, however, as they suggest that the

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y30Si of rivers increases as the fraction of Si locked up in clays decreases, the opposite from what would be predicted if clay formation is the major control on riverine y30Si (De La Rocha et al., 2000). It is clear that much more river data are needed before any assessment of glacial–interglacial variation in riverine y30Si can be carried out. 4.3. Potential for using d30Si of diatom opal to track long-term changes in the silica cycle Vast changes in the concentration of silicic acid in the ocean are thought to have occurred over the Phanerozoic, driven by the evolution of opal biomineralizing organisms (Harper and Knoll, 1975; Maliva et al., 1989; Siever, 1991; Racki and Cordey, 2000). To what extent might silicon isotopes assist in detailing when and in how many steps this drop occurred? Concentrations of silicic acid in the Precambrian, prior to the appearance of the first opal biomineralizers, were controlled by the kinetics of reactions between seawater and seafloor basalts, clays, and opal-CT and have been estimated to be as high as 1.3 mM (Siever, 1991). Over the Paleozoic, following the evolutionary radiations of siliceous sponges and radiolarians, silicic acid concentrations in the ocean dropped until they were about 1 mM, too low to fuel the abiotic precipitation of silica from ocean waters (Maliva et al., 1989), but high enough to provide a supply, through diffusion through the sediments, of silicon for the growth of opal-CT nodules (Siever, 1991). Silicic acid concentrations plummeted seriously once again following the Mesozoic appearance (Harwood and Gersonde, 1990; Sinninghe Damste´ et al., 2004) and early Tertiary radiation (Tappan and Loeblich, 1973) of the diatoms, settling sometime in the Eocene to levels near the ~70 AM average silicic acid concentration of the modern ocean (Tre´guer et al., 1995). This proposed history of the silica cycle is supported by the contemporaneous decline in the degree of silicification of sponges (Maldonado et al., 1999) and radiolarians at this time (Harper and Knoll, 1975; Racki and Cordey, 2000) and the shift in the way and under what circumstances cherts have been formed (Siever, 1991). As much as it could be hoped that diatom opal might provide a useful means with which to isotopi-

cally document their rise to dominance over the silica cycle, it does not appear that this is likely to be the case. As with the models runs of changed river fluxes during the Quaternary, runs of the 2-box model in the bpre-diatomQ ocean suggested that an imbalance between inputs and outputs of the global silica cycle must be fairly large before it is recorded in the y30Si record. In the model, a linear drop of surface silicic acid concentrations from 1000 AM down to 0 had to occur in less than a million years to shift y30Si of diatom opal by more than the +0.2x to +0.3x (Fig. 4B,E) really needed to be seen in the sedimentary record. This required a sustained output flux of silicon from the ocean that was at least 25% greater than the input flux. Pretty much the same thing was true even when opal outputs increased exponentially as they might have done in association with an exponential increase in diatom biomass, or numbers of species. Although the exponential draw down of silicic acid increased the size of the shift in y30Si in diatom opal over the drop in silicic acid, the time required for the draw down still needed to be less than a million years to produce a visibly large impact on diatom y30Si (Fig. 5B,E). When there was only a slight imbalance between the fluxes of silicon in and out of the ocean and overall silicic acid concentrations were high, the y30Si of silicic acid in surface waters (Figs. 4B and 5B) stuck very close to the steady state equilibrium value of +1.9x equalling the difference between the y30Si of the inputs (+0.8x) minus the y30Si difference between silicic acid and opal (1.1x). As a result, the y30Si of diatom opal remained steadily between + 0.8x and + 0.9x over most of the drop in silicic acid concentrations, even when the total drop was as large as 1000 AM. Even when the deep water y30Si began to decrease towards the riverine value of +0.8x at concentrations below about 200 AM (Figs. 4B and 5B), this was not mirrored in surface water y30Si. This is because Rayleigh distillation (Fig. 1) drove the surface water y30Si up even as the y30Si of its source via upwelling declined (Figs. 4B,E and 5B,E). 4.4. A refined estimate of the rate of silicic acid draw down associated with the rise of the diatoms Is it realistic for the 20-fold drop in silicic acid concentrations following the rise of the diatoms to

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have occurred in less than the 1 million years required to make a significant dent in diatom opal y30Si? We offer up one potential way of estimating how quickly oceanic silicic acid concentrations could have been drawn down by the rise of the diatoms and their resulting expansion in opal production. This is not meant to be an exact estimate, nor a recreation of history. This estimate is given merely to provide a ballpark for the rate of rise in opal production associated with the emergence of diatoms. We have assumed, for this calculation, that the amount of opal produced by diatoms is directly proportional to the increase in their numbers following their first appearance. Diatoms first appear, centric in form, in the fossil record in the Early Jurassic (Harwood and Gersonde, 1990). The subsequent radiation of the centric diatoms throughout the ocean has not been well documented. A second form of diatom, pennate in shape, emerged several tens of millions of years later, in the Paleocene (Tappan and Loeblich, 1973). We have a good sense from the sediments of the exponential increase in the number of pennate diatom species over time (Small, 1950; Fig. 6). If the apparent rate of radiation of the pennate diatoms can be taken as indicative of the ballpark rate of increase of diatom biomass (and hence opal biomass) during the initial rise of the diatoms, then

700 600

a

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60

40

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0

Age (Ma) Fig. 6. The number of known species of pennate diatom per geological age. Points are replotted from Small (1950). The curve shown is an exponential fit through the data ( y = 1.03e(6.4E8t ); R 2 = 0.94).

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we have a way of very roughly estimating the rate of depletion of silicic acid from the ocean. This estimate ignores the changes in opal preservation efficiency that would have occurred as silicic acid concentrations fell (Hurd, 1972). This estimate is only put forth as a rough minimum estimate of the amount of time required for diatoms to draw silicic acid concentrations of the ocean down to modern values. An exponential curve ( y = 1.03e6.4E  8t ; R 2 = 0.94) may be fit through the data of the number of pennate diatom species known (Small, 1950) for each geologic age (Fig. 6). This yields a rate constant of 6.4  108 yr1. When opal production in the 2-box model was increased in a similar fashion from the initial steady state value of 200 Tmol Si yr1 (i.e. BSiprod = (2.0  1014)e6.4E  8t ), the 1000 AM of silicic acid was drawn down to 0 in 2.5 million years. The associated shift in y30Si of diatom opal was small, 0.2x, and only on the brink of being observable in the sedimentary record. 4.5. A predicted transition in the d30Si of seawater silicic acid in the Cenozoic All in all, results from the 2-box model suggest that the y30Si of diatom opal will not be useful for tracking changes in the silicic acid content of seawater over the Phanerozoic as might have been driven the rise of the diatoms themselves. But the model did show a transition in the average y30Si of the ocean between the high silicic conditions of the bpre-diatomQ ocean and the low silicic acid conditions of the bdiatom dominatedQ ocean. This transition might be visible in opal produced in deep waters, such as is done by sponges (De La Rocha, 2003). When marine concentrations of silicic acid were high and the overall amount of silicic acid upwelled into the euphotic zone was also high, the 200–300 Tmol of opal produced each year to keep the system close to steady state was a small fractionation of the total silicic acid in surface waters. In this case the y30Si of diatom opal was 1.1x relative to that of surface water silicic acid and the y30Si of silicic acid remained close to the value of the deep water silicic acid replenishing it each year (e.g. Fig. 5). Because the y30Si of the opal removed from the system was approximately equal to that of the silicon input into the ocean, the average y30Si of seawater silicic acid

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was offset from the value of the inputs by the absolute value of the fractionation. If the model results can be applied to the real ocean then the average y30Si of silicic acid prior to the rise of the diatoms would have been considerably higher than the ~ +0.8x of the modern ocean. Average marine y30Si would have come in somewhere between +1.9x and +4.6x, set by the average fractionation factor for silicon isotopes. In the modern ocean, the fractionation of 1.1x of the diatoms dominates. Before the diatoms became dominant, the average silicon isotope fractionation in the ocean would have been closer to the 3.8x of sponges (De La Rocha, 2003). In an ocean with a silica cycle dominated by diatoms, both seawater silicic acid y30Si and that of sedimenting opal should be close to the roughly +0.8x value of the inputs. The reason for this convergence of y30Si values, which can be seen in natural samples (De La Rocha et al., 2000) and in the results from the 2-box model (Figs. 4 and 5), is that diatom production in the modern ocean all but completely consumes the silicic acid available in the surface ocean. The diatom opal thus takes on an average y30Si that is essentially equal to that of the deeper waters upwelled into the euphotic zone, exporting from the ocean silicon that has the same isotopic composition as average ocean waters (Wischmeyer et al., 2003). Thus at low average concentrations of silicic acid, the y30Si of silicic acid in the ocean and rivers and of diatom opal will all be very similar. This means that at some point during the transition from a bpre-diatomQ ocean to a bdiatom dominatedQ ocean, the y30Si of average marine silicic acid must have dropped from values in excess of +1.9x down to the values of roughly +0.8x in the modern ocean. This can be seen in Fig. 5 as a drop in silicic acid y30Si as concentrations of silicic acid drop below about 300 AM (Figs. 4 and 5). The drop in surface ocean y30Si is even sharper and more pronounced than it is in the deep. The reason that diatom opal did not show the ocean-wide drop in y30Si that occurred as silicic acid concentrations dropped below 300 AM is that the surface ocean, to which the production of diatom opal is confined, is a very small volume of the total ocean. The drop in the y30Si of silicic acid in the surface box is exactly compensated for by Rayleigh distillation as

opal production consumes more and more of the silicic acid available. If one were to look for the ocean-wide drop in y30Si that may have occurred as the ocean settled into its modern, low-silicic acid state, it would have to be in opal produced in the deep ocean. In this reservoir, concentrations of silicic acid are not seasonally drawn down by opal production, and the y30Si of opal produced here (even in the modern ocean) should directly parallel that of the deep water silicic acid. Spicules from siliceous sponges, which grow on the seafloor, may be the best bet for the reconstruction of a deep water record of y30Si (De La Rocha, 2003).

5. Conclusions Three conclusions can be drawn from this simple exploration of silicon isotopes in the global ocean. The first is that if the average y30Si of riverine silicic acid is relatively invariant with climate and weathering intensity, an increase in the riverine delivery of silicon to the oceans over glacial– interglacial cycles will have minimal impact on the average y30Si of sedimentary diatom opal. A dramatic decrease in the riverine delivery of silicic acid could however shift whole-ocean y30Si due to a shift in the y30Si of inputs towards lower, nonriverine values. They key issue to resolve in this respect is the extent to which global average river y30Si can change due to rates of weathering and clay formation. The second conclusion is that diatom opal is relatively insensitive to ocean-wide imbalance in silica budgets and may be of little use in tracking the overall evolution of the silica cycle over the Cenozoic. The third conclusion is that the transition of the ocean from a high silicic acid bprediatomQ condition to the low silicic acid bdiatom dominatedQ ocean of the modern day had a large impact on the y30Si of silicic acid (but not of diatom opal). The y30Si of average marine silicic acid in the bpre-diatomQ ocean was most likely greater than +1.9x and may have been as high as +4.6x. In the modern ocean, this value is only about +0.8x. Opal produced by sponges is the most likely candidate for having recorded this transition in y30Si.

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Acknowledgments We thank Adina Paytan for the invitation to participate in the special session in ocean chemistry over the Phanerozoic at GSA. We also thank Dave DeMaster, Diana Varela, and two anonymous reviewers for useful reviews.

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