Germanium cycling in the waters across a frontal zone: the Chatham Rise, New Zealand

Marine Chemistry 80 (2003) 145 – 159 www.elsevier.com/locate/marchem

Germanium cycling in the waters across a frontal zone: the Chatham Rise, New Zealand Michael J. Ellwood a,*, William A. Maher b a

National Institute of Water and Atmospheric Research, P.O. Box 11 115, Hamilton, New Zealand b Ecochemistry Laboratory, University of Canberra, Bruce ACT 2601, Canberra, Australia

Received 25 February 2002; received in revised form 16 September 2002; accepted 16 September 2002

Abstract Transect and profile data reveal that Si and Ge concentrations are depleted in the surface waters across a frontal zone to the east of New Zealand. These results are consistent with Si and Ge uptake and regeneration from siliceous organisms. The Ge/Si ratio along three transects is not constant with Si utilisation indicating that there is fractionation between Si and Ge during uptake by phytoplankton. A fractionation factor (KD) of 0.36 is obtained from the transect data for Si concentrations below about 6 AM, assuming a Rayleigh distillation-like process. Although Si utilisation is slight faster than that of Ge, assuming that the Si(OH)4 and Ge(OH)4 are the chemical species utilised, such chemical fractionation is unlikely to contribute to the variations seen in the Ge/Si transect data, rather biological fractionation appears to dominate. Profiles for Ge/Si versus depth reveals a subsurface maximum in the Ge/Si data suggesting that Ge is being recycled faster than Si from phytoplankton. Such Ge/Si fractionation during Si and Ge uptake and regeneration is the most likely explanation for the positive Ge intercept seen for the global Ge versus Si relationship. Biological fractionation of Ge is contrary to the results of Bareille et al. [Geology 26 (1998) 82], who observed little variation in Ge/Siopal values for diatom frustules isolated from sediments along a transect where a strong Si concentration gradient exists. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Ge; Uptake; Phytoplankton

1. Introduction In seawater, the uptake and regeneration profile of Ge mimics that of Si (Froelich and Andreae, 1981; Froelich et al., 1989; Mortlock and Froelich, 1996). The depletion of both Si and Ge in the surface ocean is primarily by diatoms, which used Si for opal

* Corresponding author. Tel.: +64-7-856-7026; fax: +64-7-8560151. E-mail address: [email protected] (M.J. Ellwood).

formation. Upon death, diatoms sink out of the euphotic zone and their frustules either dissolve or deposit in the sediments below. In contrast to the nutrient-like profile for inorganic Ge, the profiles for monomethyl and dimethyl germanium (MMGe and DMGe) are conservative (Lewis et al., 1985, 1988, 1989; Santosa et al., 1997). The conservative distribution of MMGe and DMGe in the ocean indicates that they are neither significantly produced by, nor removed by, marine organisms. Indeed, the stability of both MMGe and DMGe is highlighted by their lack of degradation to inorganic Ge when irradiated with

0304-4203/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 0 3 ( 0 2 ) 0 0 11 5 - 9

146

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

strong UV light (Lewis et al., 1985). The major sources of MMGe and DMGe are as yet undiscovered, but are thought to be terrestrial in their origin, whereas the major sources of inorganic Ge to the oceans are from riverine, atmospheric and hydrothermal sources (Froelich et al., 1985; Lewis et al., 1989; Santosa et al., 1997; Hammond et al., 2000; King et al., 2000). Although the covariation of Ge with Si is highly correlated, there is a positive and significant Ge intercept for the Ge-to-Si relationship (Froelich and Andreae, 1981; Froelich et al., 1985, 1989; Murnane and Stallard, 1988; Mortlock and Froelich, 1996). The intercept value varies between 1.7 and 3.6 pM depending on the analytical system used for Ge detection. Early methods used to measure Ge in seawater had detection limits of f 4 pM (Froelich et al., 1989), whereas more modern analytical systems have detection limits in the subpicomolar range (0.1 –0.4 pM) (Jin et al., 1991; Mortlock and Froelich, 1996; Ellwood and Maher, 2002). Although modern analytical systems have lower Ge blank values and detection limits, the positive Ge intercept still remains (Mortlock and Froelich, 1996; Santosa et al., 1997). The positive Ge intercept is thought to result from Ge/Si fractionation during Si uptake (Murnane and Stallard, 1988; Froelich et al., 1989). However, it is unclear as to where exactly this fractionation is occurring within the diatoms. Uptake studies involving Ge radioisotopes indicate that at low Ge/Si ratios ( < 0.01) up to 80% of Ge is incorporated into the cell wall of the diatom (Azam et al., 1973, 1974). In contrast, Mehard et al. (1974) found that there is some fractionation of Ge and Si between different cell organelles. To fully understand the uptake and incorporation of Ge into diatom frustules requires knowledge of Si uptake and deposition processes. The uptake kinetics of Si by diatoms can be described using an equation similar to the Michaelis– Menten equation for enzyme kinetics: V ¼ Vmax

½SiðOHÞ4  ðKm þ ½SiðOHÞ4 Þ

ð1Þ

where Vmax is the maximum rate for Si uptake and Km is the half-saturation constant for Si(OH)4 concentration at half Vmax, i.e. V = 1/2Vmax. Although the overall consumption of Si by diatoms from seawater can be described using this type of equation, the uptake

and deposition of Si in diatom frustules is a multistep process. A conceptual model for Si uptake and deposition has been presented by Ragueneau et al. (2000, and references therein). In this model, the first step in frustule formation involves the active transport of Si across the cell membrane via silicon transporters (Bhattacharyya and Volcani, 1980, 1983; Hildebrand et al., 1997, 1998) into a soluble Si pool. Silicon uptake occurs prior to cell division and valve formation. The size of the Si pool varies depending on the specific phase in the cell cycle and can contain up to 120% of the cells requirement prior to dividing (Brzezinski and Conley, 1994). Loss of Si from dissolved internal pools, i.e. Si leakage, has also been noted using 68Ge(OH)4 as a tracer for Si(OH)4 efflux (Sullivan, 1976). The rate of Si uptake by diatoms is also dependent on the chemical form of Si (Del Amo and Brzezinski, 1999). Kinetic studies on the uptake of Si(OH)4 and SiO(OH)3 suggest that the majority of Si taken up by marine diatoms is Si(OH)4. At a seawater pH of 8.1, and using a pKa of 9.82 (Busey and Mesmer, 1977), about 98% of Si is estimated to be as Si(OH)4.. Once accumulated internally within the soluble pool, Si is then used in metabolic processes and in frustule formation (Lewin, 1955; Darby and Volcani, 1969; Sullivan and Volcani, 1973). The uptake and incorporation of Ge in diatoms is thought to follow a similar pathway to that of Si (Azam, 1974). The incorporation of Ge into diatom opal has been used by Froelich et al. (1989, 1992) and Mortlock et al. (1991) to reconstruct changes in the Si water column cycle over time. The Ge/Si signature of diatom opal (Ge/Siopal) from the past 450,000 years reveals systematic changes that are coherent with glacial– interglacial cycles (Mortlock et al., 1991). Interpretation of the Ge/Siopal record has varied. Initially, it was thought that the decrease in the Ge/ Siopal record from an interglacial value of 0.72 Amol mol 1 to a glacial value of 0.55 Amol mol 1 indicated a reduction in Ge uptake relative to Si as a result of lower Si utilisation during glacial times (Froelich et al., 1989; Mortlock et al., 1991). However, diatom growth experiments have shown that diatoms grown at high Si concentrations (100 AM) do not fractionate Ge/Si significantly leading Froelich et al. (1992) to suggest that glacial ocean Si concentrations were higher than the present day.

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

Clearly, there is a need to understand the processes involved in the uptake and incorporation of Ge into biogenic opal before the Ge/Siopal record can be correctly interpreted. A positive intercept for the Ge versus Si seawater relationship would indicate that Ge – Si fractionation is occurring. Such fractionation processes should be readily apparent in the surface ocean, where diatoms grow and Si utilisation occurs. In this paper, we present measurements of Ge and Si concentrations along three surface seawater transects and down three profile stations across a frontal zone east of New Zealand, in an attempt to model the uptake of Ge from these waters. If fractionation processes do dominate Ge and Si uptake and regener-

147

ation, variations in the seawater Ge-to-Si relationship should be readily apparent in this oceanic region were Si concentrations can reach extremely low levels, especially over the Chatham Rise.

2. Oceanographic setting The Chatham Rise to east of New Zealand is a dynamic region, where southward moving warm subtropical water mixes with northward moving cooler subantarctic water. The Chatham Rise is an oceanic region where the seafloor rises sharply from a depth of about f 3000 m to a depth of f 300 m and declines

Fig. 1. Map of transects and profile stations (Bx24, Bx64 and CS5) for Box Cruise TAN0010.

148

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

Fig. 2. Transects showing (A) seafloor depth, (B) salinity (x), (C) temperature and (D) chlorophyll a concentration.

again to f 3000 m within 3j of latitude (Figs. 1 and 2A). The subtropical convergence (STC) is situated over the Chatham Rise and is the mixing zone for cool ( < 15 jC), nutrient-rich subantarctic surface water (SASW) (salinity = 34.5x ) with warm (>15 jC) nutrient-depleted subtropical water (STW) (salinity = 35.7x ). The macronutrient signatures of these two water masses are quite different. Subtropical waters are characteristically low in NO3 (0.7 AM), Si (1.3 AM) and PO4 (0.3 AM), whereas the opposite is true of subantarctic waters, NO3 (18 AM), Si (4 AM) and PO4 (1.4 AM) (Bradford-Grieve et al., 1997; Boyd et al., 1999).

3. Methods 3.1. Sample collection Seawater samples were collected on a 15-day cruise in October 2000 on the R/V Tangaroa (TAN0010, Box Cruise). Along three transects, surface water samples were collected using a towed

‘‘fish’’ system similar to that described by Bowie et al. (2001). Briefly, surface seawater was pumped from a water depth of f 3 m into a shipping container equipped with a trace-metal clean sample handling area while the vessel was steaming. This handling area consisted of a laminar flow bench within a plastic clean tent. On-line filtration of samples was carried out by pumping individual samples through acidcleaned Sartobran-P capsules (0.45-Am prefilter, 0.2Am final filter) (Sartorius, Germany). Deep-water samples were collected using 5-l acidcleaned Teflon-lined Go-Flo bottles (General Oceanics, USA) deployed on a Kevlar line. Go-Flo bottles were tripped with solid polyvinyl chloride messengers. Upon recovery, water samples were drawn from the Go-Flo bottles inside the laminar flow bench and filtered through acid-cleaned Sartobran-P capsules (0.45-Am prefilter, 0.2-Am final filter) (Sartorius). Filtered samples were stored in either 250- or 500ml acid-cleaned high-density polyethylene bottles (Nalgene, USA). Samples were acidified to a pH < 2 with quartz-distilled HCl (q-HCl) and stored doublebagged at 4 jC prior to analysis.

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

3.2. Analysis The analytical procedure used to determine Ge, MMGe and DMGe combines the principles of flow injection and batch hydride generation analysis (Ellwood and Maher, 2002). Briefly, Ge species are reduced with sodium tetrahydroborate to their corresponding hydrides which are then trapped at liquid nitrogen temperature in a Teflon U-tube packed with 10 cm of 5% SE-30 on Chromosorb W-HP. Individual Ge hydride species were evolved by removing the Utube from the liquid nitrogen dewar and heating. Chromatographic separation of Ge hydride species is by their boiling points and interaction with the Chromosorb solid phase. Ge hydrides were detected by ICP-MS (ELAN 6000, Perkin Elmer, USA). Prior to analysis, 40-ml aliquots of samples were measured out into acid-cleaned Teflon tubes, further acidified to 0.06 M with q-HCl and spiked with trimethyl germanium (TMGe) (100 pM) as an internal standard (Mortlock and Froelich, 1996). Sample concentrations were determined using peak area against standard calibration curves.

149

Inorganic Ge and methylated Ge species were determined in separate analyses. Inorganic Ge was determined using a 10-ml sample loop and without cysteine being added to the samples. Cysteine was not added because it was found to contain Ge (6.5 F 0.8 pM). The addition of cysteine does not assist in the formation of Ge hydride, only the formation of MMGe and DMGe hydrides (Ellwood and Maher, 2002). Without cysteine, the overall analytical blank for inorganic Ge was 0.6 F 0.1 pM. This blank almost entirely came from the 0.5% w/v sodium tetrahydroborate solution used to generate hydrides and not from the q-HCl used to acidify samples, blanks and standards. Note that all samples concentrations presented in this paper are blank corrected values. MMGe and DMGe species were determined using a 1-ml sample loop and with 0.5% w/v of cysteine added to samples to aid MMGe and DMGe hydride formation. The detection limit for inorganic Ge is 0.1 pM using a 10-ml sample loop and 5 and 1 pM for MMGe and DMGe, respectively, using a 1-ml sample loop. Method reproducibility (RSD) was 2%, 4% and 3%

Fig. 3. Concentrations of (A) PO4, (B) Si, (C) Ge and (D) Ge/Si along the three transects. Dotted lines indicate the seawater Ge/Si ratio of 0.724 Amol mol 1 (Mortlock and Froelich, 1996).

150

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

for inorganic Ge, MMGe and DMGe at an analyte concentration of 6, 350 and 100 pM, respectively (n = 5).

4. Results

3.3. Macronutrients analysis

4.1.1. Physical parameters, nutrients and chlorophyll a Across the Chatham Rise, there was a large gradient in temperature and salinity with warm more saline STW north of the Chatham Rise and cooler less saline SASW south of the Chatham Rise (Fig. 2B and C). Nutrient concentrations also varied along the transects (Fig. 3A and B, Table 1). Along the Hawkes Bay transect, PO4 concentrations are relatively constant whereas Si decreases moving southeast. Along the Chatham Rise transect, PO4 concentrations increased southward along the transect from 0.3– 0.4 to 0.95 AM south of 44 jS. Si concentrations were low along the Chatham Rise transect with concentrations ranging between 0.5 and 1.5 AM at the northern and southern ends of the transect, respectively. On the Chatham Rise, Si concentrations drop below the detection limits (0.07

Macronutrients were determined immediately after collection on aliquots of filtered samples using an Astoria Analyzer 300 series autoanalyser (AstoriaPacific International). Dissolved reactive phosphorus was determined by the autoanalytical method of Downes (1978) modified by eliminating the reductant. Silicic acid was determined using the method of Smith and Milne (1981), and nitrite plus nitrate were determined using the method developed by Nydahl (1976). Silicic acid detection limit was 0.07 AM and reproducibility was 0.18 AM. Duplicate chlorophyll a samples were collected on GF/F filters and folded into aluminium foil and immediately frozen. Ashore, chlorophyll a was extracted into 90% acetone and determined by fluorometry (Strickland and Parsons, 1972).

4.1. Surface transects

Table 1 Ge, MMGe, DMGe, Si and PO4 concentrations for surface transect samples collected between 8/10/2000 and 19/10/2000 Latitude

Longitude

Ge (pM)

MMGe (pM)

DMGe (pM)

Si (AM)a

PO4 (AM)

45j49.9VS 45j55.4VS 46j02.1VS 46j08.6VS 46j14.4VS 46j24.5VS 46j30.3VS 45j21.0VS 44j33.5VS 44j00.0VS 43j41.2VS 43j15.7VS 43j03.7VS 42j52.1VS 42j44.9VS 42j16.9VS 41j38.8VS 41j11.1VS 40j26.7VS 40j12.3VS 40j02.9VS 39j49.7VS

171j39.6VE 172j27.8VE 173j32.7VE 174j51.2VE 175j48.9VE 177j15.2VE 178j30.0VE 178j29.8VE 178j36.0VE 178j29.4VE 178j30.1VE 178j29.3VE 179j29.5VE 178j29.8VE 178j29.8VE 178j31.2VE 178j30.7VE 178j28.9VE 178j09.5VE 177j50.4VE 177j38.2VE 177j20.0VE

2.9 2.5 1.6 1.7 1.9 2.4 2.1 1.3 2.0 1.0 0.8 0.7 0.7 2.1 1.1 2.3 1.6 2.6 1.6 2.6 2.4 3.9

363 345 324 348 361 356 362 371 356 359 347 367 347 350 370 359 364 367 357 365 363 358

97 100 104 101 92 101 99 99 100 100 101 103 92 103 100 103 106 107 99 105 116 106

4.54 3.96 3.13 0.97 1.11 1.04 1.32 1.18 1.56 0.35 < 0.07 < 0.07 < 0.07 0.86 < 0.07 0.77 0.79 0.94 0.51 0.58 0.71 1.72

1.02 1.11 0.96 0.88 0.95 0.95 0.95 0.95 0.96 0.61 0.48 0.40 0.41 0.45 0.38 0.30 0.27 0.31 0.30 0.25 0.28 0.33

a

Detection limit for Si determinations was 0.07 AM.

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

AM) indicating significant Si utilisation by the phytoplankton community. Along the Otago transect, PO4 concentrations remain relatively constant at f 1 AM whereas Si concentrations increased steadily from 175jE onwards, moving towards the Otago coast. Chlorophyll a concentrations along the Hawkes Bay transect ranged between 0.72 and 0.89 mg m 3 (Fig. 2D). Along the Chatham Rise transect, chlorophyll a concentrations decrease from 0.41 to 0.29 mg m 3 in a southward direction until on the Chatham Rise, where values jump to f 0.75 mg m 3 and then decrease to f 0.22 mg m 3 at f 45jS. The increase in chlorophyll a concentrations on the Chatham Rise accompanies the low Si concentrations, indicating Si consumption by phytoplankton. Along the Otago transect, chlorophyll a concentrations remain comparatively constant with values ranging between 0.08 and 0.24 mg m 3.

151

Bay transect increased slightly from about 2.5 to 3.5 Amol mol 1 moving southeast (Fig. 3D). These ratios are somewhat higher than the Ge/Si ratio of 0.724 Amol mol 1 obtained by Mortlock and Froelich (1996). Along the Chatham Rise transect, Ge/Si values increase from f 3 to 10– 15 Amol mol 1 on top of the Chatham Rise where Si concentrations are extremely low. South of the Chatham Rise, Ge/Si values decrease to between 1 and 2 Amol mol 1. Along the Otago transect, Ge/Si values decrease from 1.5– 2 to 0.6 –0.7 Amol mol – 1 nearer the Otago coastline. MMGe and DMGe concentrations did not vary significantly over the three transects with an average concentration of 357 F 11 and 102 F 5 pM, respectively (Table 1). The concentrations for these two methylated Ge species are similar to the ranges reported for the Atlantic, Pacific and Antarctic Oceans (Lewis et al., 1985, 1989). 4.2. Vertical profiles

4.1.2. Germanium Germanium concentrations along all three transects parallels that of Si with low values ( f 0.7 pM) measured on the Chatham Rise (Fig. 3C, Table 1). These values are the lowest so far reported for openocean surface waters. Ge/Si values along the Hawkes

4.2.1. Nutrients The nutrient and chlorophyll a profiles obtained at the subtropical (Bx64), subantarctic (Bx24) and Cook Strait (CS5) stations (Fig. 4, Table 2) are typical those seen for this oceanic region. Both PO4 and NO2 + NO3

Fig. 4. Profiles of (A) PO4, (B) Si and (C) chlorophyll a concentrations versus depth.

152

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

Table 2 Ge, MMGe, DMGe, Si and PO4 concentrations for profile stations occupied between 8/10/2000 and 19/10/2000

lock and Froelich, 1996). A linear best-fit line through the profile data yields a line of:

Depth (m)

Ge ðpMÞ ¼ 0:796F0:036 Si ðAMÞ þ 1:39F0:31;

Ge (pM)

MMGe (pM)

DMGe (pM)

Si (AM)

PO4 (AM)

1.27 1.27 1.15 1.12 4.75 11.33 32.65

1.06 1.06 0.94 0.98 1.34 2.30 2.57

178830.0VE 104 0.80 100 0.79 106 0.84 100 1.04 99 0.84 97 2.88 95 6.26 96 10.51

0.31 0.32 0.33 0.33 0.51 1.12 1.41 1.62

Subantarctic station (Bx24), 46830.0VS, 178830.0VE 25 1.2 331 108 50 1.3 329 105 100 1.7 324 106 200 1.7 296 100 400 4.4 304 100 580 9.2 304 100 980 26.6 315 105 Subtropical station (Bx64), 41800.0VS, 25 1.6 352 50 1.4 329 100 2.2 328 200 1.7 326 400 2.4 321 600 5.3 331 800 7.3 331 1000 12.9 292

Cook Strait (CS5), 41838.6VS, 174858.0VE 25 3.4 383 109 50 2.3 354 107 100 4.2 357 95 400 3.3 363 102 600 6.3 360 92

1.95 1.91 1.55 3.09 5.26

0.39 0.46 0.45 0.99 1.25

ðr2 ¼ 0:964; n ¼ 20Þ with an overall standard error of 1.16. Methylated Ge species concentration profiles varied little with average concentrations of 331 F 24 pM (means F S.D.) and 102 F 5 pM for MMGe and DMGe, respectively (Fig. 5B, Table 2). A plot of Ge/Si versus depth reveals a subsurface maximum in the Ge/Si data between about 50 and 500 m for all three stations (Fig. 5C). Ge/Si values of about 1.5 Amol mol 1 are observed for the subantarctic station and 2.5– 3 Amol mol 1 for the subtropical and Cook Strait stations. The increase in Ge/Si values for the subtropical station versus the subantarctic station coincides with increased chlorophyll a concentrations for this station, suggesting that the increase in Ge/Si values between the surface and 100 m results from biological processes. The subsurface maximum also indicates that Ge is being recycled faster than Si in the waters below the primary production zone. 4.3. Germanium versus silicon: all data

(data not presented) concentrations are higher for subantarctic waters compared with subtropical and Cook Strait waters reflecting a north – south gradient of increasing nutrient concentration. Conversely, chlorophyll a concentrations are higher for subtropical waters compared to subantarctic waters. Surface water Si concentrations were low at all three stations and increased with depth below about 200 m (Fig. 4B). 4.2.2. Germanium The vertical Ge concentrations profiles presented in Fig. 5A and in Table 2 are typical of those measured for this element in other oceanic regions (Froelich et al., 1985). The profiles show surfacewater depletion and enrichment with depth. For the first 250 m, Ge concentrations are fairly constant after which they increase with depth in manner similar to that of Si (Fig. 4B). The close similarity between the Ge and Si profiles indicates that Ge and Si is being taken up and regenerated by similar processes (Froelich and Andreae, 1981; Froelich et al., 1985; Mort-

A linear best-fit line through all the Ge versus Si data yields the following equation: Ge ðpMÞ ¼ 0:797F0:030 SiðAMÞ þ 1:15F0:18; ðr2 ¼ 0:946; n ¼ 42Þ and with an overall standard error of 1.02. We tested whether the intercept was significant using a onesided t-test and obtained a t value of 6.43 which is higher than t40,0.05(1), ( P < 0.0001). Thus, the intercept is positive and significant at the 95% confidence level. Our original hypothesis, before samples were collected, was that there was a positive intercept, hence, the use of a one-sided t-test. The Ge versus Si slope of 0.797 F 0.030 obtained in this study is slightly higher than the slope of 0.724 F 0.004 obtained by Mortlock and Froelich (1996). However, the concentration range here is much smaller than in their study. The intercept (1.15 F 0.18 pM) obtained for the Ge versus Si relationship is also

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

153

Fig. 5. Profiles of (A) Ge and (B) MMGe and DMGe concentrations versus depth. (C) Ge/Si versus depth.

lower than the intercept (1.74 F 0.23 pM) obtained by Mortlock and Froelich (1996).

5. Discussion The cycling of inorganic Ge in the ocean is dominated by its uptake and regeneration from plankton, mainly diatoms (Froelich and Andreae, 1981; Froelich et al., 1992). Although the Ge cycle is tightly coupled to the Si cycle, a positive intercept is observed when inorganic Ge concentration is plotted against Si concentration, indicating that some fractionation of Ge is occurring (Murnane and Stallard, 1988; Froelich et al., 1989). Such fractionation is akin to isotopic discrimination of 30Si relative to 28Si during Si uptake and regeneration by diatoms (De la Rocha et al., 1997, 2000). When the Ge/Si transect data is plotted against Si concentration clear curvature is seen at low Si concentrations suggesting a Rayleigh fractionation-like process (Fig. 6A). Below about 5 AM, the Ge/Si increases significantly from a value of f 0.7– 0.8 Amol mol 1 to reach values of f 15 Amol mol 1 at a Si concentration of about 0.1 AM. It should be noted that a

straight line with a positive intercept would also produce curvature at low denominator values when the ratio is plotted against one of its values, i.e. y/x versus x. However, the question is whether Ge is being fractionated like an isotope of Si or whether it is resultant from different Ge and Si input sources. If the positive intercept is resultant from biological fractionation, then it is valid to plot Ge/Si versus Si and invoke a Rayleigh-like distillation process. The two likely candidates for the positive Ge intercept seen in the Ge versus Si concentration data set are: (a) dust input to the Chatham Rise region thereby increasing Ge concentrations relative to Si and (b) biological fractionation for Ge/Si during uptake and regeneration. 5.1. Dust inputs If atmospheric dust inputs to the surface ocean are significant, then this may well explain the positive intercept seen in the Ge versus Si data. Continental derived particles typically have Ge/Si ratios between about 1.3– 2.5 (Amol mol 1), which is about two to four times higher than oceanic ratios (Mortlock and Froelich, 1987). The input of atmospheric dust into

154

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

not the cause for the positive intercept seen in the Ge/ Si data set. If dust inputs were a significant contributor of Ge, higher Ge/Si values at the ocean surface would be expected and this was not observed in our data. Regions where dust input might affect Ge/Si ratios are the North Atlantic and the North Pacific, where dust inputs are 5 –12 times higher and surface nutrient concentrations are low (Duce et al., 1991; Wu et al., 2000). Clearly, dust input into the waters east of New Zealand is unlikely to affect Ge and Si cycling, thus, the most likely cause of the positive intercept seen in the Ge/Si data set is from biological fractionation during uptake and regeneration. 5.2. Biological fractionation To model the fractionation of Ge, we treated the discrimination of Ge relative to Si as a Rayleigh distillation-like process, i.e. the Ge/Si ratio is treated in a similar fashion to an isotope ratio (Froelich et al., 1992) as defined by: KD ¼

Fig. 6. Plots of Ge/Si versus Si for (A) transect data and model curves generated assuming Ge discrimination is as a Rayleigh distillation process (Eq. (2)). (B) Data from Froelich et al. (1992) for Thalassiosria antarctica culture no. 1 and from Azam (1974, Fig. 2). The 68Ge/31Si data from Azam (1974) has been normalised to the Froelich data by multiplying by 16,000. Model curve for comparison was calculated using Eq. (3) assuming KD = 0.5, (Ge/ Si)DW = 22 and Sii = 20 AM.

Southern Pacific waters is low with an average Si flux of 3.9 mmol m 2 year 1 (Duce et al., 1991). If we assume that this is typical for the Chatham Rise region, this would equate to a Ge flux of 9.75 nmol m 2 year 1, using a Ge/Si ratio of 2.5 Amol mol 1. Assuming a Ge particle solubility of 10% (as for Si) (Duce et al., 1991), this in turn would only increase inorganic Ge concentrations over one year by about 98 fM if mixed to 10 m. This low dust derived Ge concentration coupled with the subsurface maximum seen for the Ge and Si data would suggest that atmospheric inputs to the Chatham Rise region are

ðGe=SiÞpart: ðGe=SiÞSW

ð2Þ

where (Ge/Si)SW is the instantaneous Ge/Si ratio in surface waters, (Ge/Si)part. is the accumulated Ge/Si ratio of particulate matter and KD is the fractionation factor between the water and particulate phases. The change in the surface waters Ge/Si values during Ge and Si uptake can be modelled using the following Rayleigh equation:     Ge Ge ¼ f ðKD1 Þ ð3Þ Si SW Si DW where Ge/SiDW is the initial Ge/Si ratio prior to fractionation. A Ge/Si value of 0.724 Amol mol 1 was assigned to (Ge/Si)DW because it is more representative of oceanic Ge/Si values than the 0.797 Amol mol 1 we obtained (Mortlock and Froelich, 1996). f is the fraction of initial dissolved Si remaining in solution, i.e. f=[Si]f/[Si]i where [Si]f is the dissolved Si concentration during uptake and [Si]i is the initial Si concentration. Eq. (2) was fitted, using a nonlinear regression method with a floating [Si]i, to the surface transect data. Only the transect data was use because

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

Si uptake by phytoplankton only occurs at or near the surface ocean. (Fig. 6A). Values obtained for the fit were 0.36 F 0.05 for KD and r2 = 0.92, with [Si]i equal to 6 AM. The KD value obtained here is close to the value 0.5 F 0.2 obtained by Murnane and Stallard, (1988) Froelich et al. (1989). A K D value of 0.36 F 0.05 indicates that Ge is significantly discriminated against during Si uptake when Si concentrations were below about 5 AM. If there was no discrimination of Ge, KD would equal one and there would be little variation in the Ge/Si data (Fig. 6A). This observation of Ge fractionation is in accordance with the results from three separate diatom culture studies (Azam, 1974; Mehard et al., 1974; Froelich et al., 1992). Azam (1974) found that the uptake rates for Ge and Si were similar until the Si concentration of the culture medium was reduced to about 5 AM (Azam, 1974, Fig. 2). At 5 AM Si, the concentration of Ge remaining in the culture medium was significantly higher than that of Si. When the data from the Azam (1974) study is converted to a Ge/Si versus Si concentration plot, an increase is seen in the Ge/Si data at low Si concentrations, similar to that observed for the surface transect data (Fig. 6B). Mehard et al. (1974) found that the uptake kinetics for Ge and Si were similar, however, the incorporation rate for Ge into trichloroacetic acid insoluble material, presumably cell walls, was much slower than that for Si. This observation is accordance with Azam (1974) in which there was significant Ge leakage/dissolution from cells (Fig. 6B). Froelich et al. (1992) investigated the uptake of Ge by two diatoms species grown at relatively high Si concentrations. Results from most of their experiments suggest that diatoms do not significantly discriminate against Ge during opal formation. However, in most of their experiments, the Si concentration in the culture medium was never fully consumed, i.e. the Si concentrations remained above 18 AM. In the experiment where Si was fully consumed, fractionation was observed, i.e. the Ge/Si water ratio increased from f 22 Amol mol 1 at Si concentrations greater than f 10 AM to 115 Amol mol 1 at a Si concentration of 0.7 AM (Fig. 6B). Taken together, the results from these studies would suggest that Ge/Si fractionation occurs in the intercellular Si pool within diatoms.

155

We also investigated whether the chemical species of Si and Ge taken up by diatoms can account for the variations seen in the Ge/Si data. Del Amo and Brzezinski (1999) have shown that the main Si species taken up by diatoms is Si(OH)4. At an oceanic pH of 8.1, approximately 98% of Si exists as Si(OH)4, and for Ge about 94% is as Ge(OH)4 using a pKa of 9.3 (Pokrovski and Schott, 1998). The difference between Si as Si(OH)4 and Ge as Ge(OH)4 is small relative to total Si and Ge concentrations, thus, it is unlikely to be a source of variation in Ge/Si ratios. Although our data suggests that there is fractionation within the diatom, especially at low Si concentrations, there does not appear to be significant curvature in the global Ge to Si data set (Murnane et al., 1989; Froelich et al., 1992). Such curvature might be expected if Ge and Si are fractionated during opal formation. If, however, the majority of discrimination only occurs in the soft part of the diatom, as suggested by the leakage of 68Ge from cells in Azam’s (1974) culture experiments and in the subsurface maximum seen in the Ge/Si profile data (Fig. 5C), then curvature should be apparent in the upper 1500 m where soft tissue elements regenerate and in young nutrientdepleted deep waters, such as those of the North Atlantic. Regions where there are high surface-water nutrient concentrations, as in the Southern Ocean, such curvature will tend to be camouflaged. We further investigated the subsurface maximum in the Ge/Si data using an approach similar to that used by Elderfield and Rickaby (2000) to model Cd and P oceanic data. Using the Rayleigh equation for integrated solids:     Ge Ge ð1  f KD Þ ð5Þ ¼ Si part: Si DW ð1  f Þ and combining with Eq. (2) gives:     Ge Ge 1 ð1  f KD Þ ¼ Si SW Si DW KD ð1  f Þ

ð6Þ

Using (Ge/Si)DW = 0.724 Amol mol 1, KD = 0.36 (obtained from the transect model data) and an average Si concentration of 60 AM, which is the Si concentration at around 1500 m (Pickmere, unpublished data), we calculated an expected Ge versus Si curve and Ge/Si versus depth curves for the suban-

156

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

tarctic, subtropical and Cook Strait stations (Fig. 7A and B). A Si value of 60 AM was chosen because this corresponded to the depth ( f 1500 m) at which PO4 concentrations stop increasing, thereby indicating that remineralisation of organic tissues is reduced or masked by high nutrient concentrations. As shown in the Ge versus Si plot, the model predicts curvature in the Ge versus Si data and easily covers the majority of the profile data presented. However, above about 10 AM of Si, Ge data is scarce so it is difficult to full assess whether the model agrees with oceanic measurements. With this in mind, we applied the model to each profile station (Fig. 7B). Clear differences between each oceanic profile becomes apparent when the model is applied; with reasonable agreement between model and measured Ge/Si values for the subantarctic station. However, for the subtropical station and the Cook Strait stations the model significantly underestimates Ge/Si values in the upper water column. Nevertheless, the model does describe gross fractionation processes for Ge and Si regeneration, but it is limited somewhat in describing such processes in detail. Overall the model highlights Ge/Si fractionation during soft tissue regeneration. One problem with invoking Ge to Si fractionation during Si uptake and regeneration is that the Ge/Siopal record should also reflect such fractionation. Bareille et al. (1998) reported Ge/Siopal measurements for Holocene samples along a transect traversing the Antarctic Polar Front (APF), where dissolved Si

concentrations increase from about 2 AM north of the APF to about 60 AM south of the APF, and found little variation in Ge/Siopal values. Indeed, Ge/Siopal values were close to the overall seawater Ge/Si ratio of 0.7 Amol mol 1. At present, we have no explanation to reconcile the lack of variation in the Ge/Siopal record along this transect with the hypothesis of Ge/Si fractionation during Si uptake and regeneration. However, differences in Ge/Siopal values between small and large diatoms of the same age and origin has been observed (Shemesh et al., 1989). Generally, the larger diatoms have lower Ge/Siopal values than smaller diatoms. Why larger diatoms should have a lower Ge/Si signature than their smaller counterparts is not known, but it does indicate that certain diatoms, large ones at least, do discriminate against Ge during opal formation. As mentioned, the Ge/Siopal record for the upper Pleistocene and Holocene shows large changes, with Ge/Siopal varying from f 0.72 Amol mol 1 for interglacial periods to f 0.55 Amol mol 1 during glacial periods (Mortlock et al., 1991). Complicating the interpretation of this record is the potential role trace metals, such as Fe, might have on fractionation processes, assuming that such processes are important in controlling Ge incorporation into biogenic opal. Hutchins and Bruland (1998) and Takeda (1998) have shown that diatoms grown under Fe-limiting conditions produce thicker more heavily silicified frustules than diatoms grown under Fe-replete conditions. The

Fig. 7. (A) Property – property plot of Ge versus Si. Straight line is the least-squares regression line for the profile data. Symbols are the same as in Fig. 5. (B) Ge/Si versus depth for each station. Model curves were generated using Eq. (6) with KD = 0.36, (Ge/Si)DW = 0.724 and Sii = 60 AM.

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

majority of the present-day Southern Ocean is thought to be Fe-limited (Martin et al., 1990; de Baar et al., 1995; Boyd et al., 2000). However, during glacial times more Fe is thought to have been deposited to the oceans through dust blown off the continents, thereby increasing surface water Fe concentrations and alleviating chronic Fe limitation (Kumar et al., 1995; Petit et al., 1999). Such changes in the glacial ocean Fe supply may well have lead to thinner diatom frustules compared to the present-day diatom frustules, which in turn would increase the recycling of Si (and presumably Ge) in the glacial ocean, i.e. less Si per diatom would have been deposited to the sea floor (Boyle, 1998; Hutchins and Bruland, 1998; Takeda, 1998). The changes seen in the Ge/Siopal record may well reflect changes in Si consumption and increased recycling of Ge and Si during glacial times. More work is clearly needed to determine to what extent diatoms fractionate Ge and Si, especially when grown in high and low trace metal regimes.

6. Conclusions Transect and profile data for Si and Ge are consistent with Si and Ge uptake and regeneration from siliceous organisms. The surface water Ge/Si data are not constant with Si utilisation, suggesting that there is significant fractionation between Ge and Si during uptake and regeneration. Variations resulting from differences in biological forms of Si and Ge during Si uptake are probably unimportant and cannot be used to explain the increase in Ge/Si data during Si utilisation. A subsurface maximum is also observed in the Ge/Si data suggesting Ge is regenerating faster than that of Si during organic tissue decay. Modelling of the transect and profile data suggests that Ge – Si fractionation occurs in the ‘‘soft’’ parts of diatoms. If Ge fractionation in soft tissues is translated to biogenic opal, the variations seen in the Ge/Siopal record for the last 450,000 years could well be linked to changes in Si utilisation.

Acknowledgements We thank Phil Boyd for inviting M.J.E. on the cruise. We also thank Stu Pickrmere for the nutrient

157

analysis, Frank Krikowa for the help in the laboratory, Russell Frew for the use of his sampling equipment, Anthony Butch for his help during sampling, the Officers and crew of the RV Tangaroa and Graham McBride for advice. Finally, we thank three referees for comments that helped to improve the manuscript. Cruise TAN0010 forms part of NIWA’s Ocean Ecosystems Programme, which is supported through a Foundation for Research, Science and Technology grant (C01X0027). Associate editor: Dr. Edward Boyle.

References Azam, F., 1974. Silicic acid uptake in diatoms studies with [68Ge] germanic acid as tracer. Planta 121, 205 – 212. Azam, F., Hemmingsens, B.B., Volcani, B.E., 1973. Germanium incoporation into the silica of diatom cell walls. Archiv fur Mikrobiologie 92, 11 – 20. Bareille, G., Labracherie, M., Mortlock, R.A., Maier-Reimer, E., Froelich, P.N., 1998. A test of (Ge/Si)opal as a paleorecorder of (Ge/Si)seawater. Geology 26, 179 – 182. Bhattacharyya, P., Volcani, B.E., 1980. Sodium-dependent silicate transport in the apochlorotic marine diatom Nitzschia alba. Marine Chemistry, 6386 – 6390. Bhattacharyya, P., Volcani, B.E., 1983. Isolation of silicate ionophore(s) from the apochlorotic diatom Nitzschia alba. Biochemical and Biophysical Research Communications 114, 365 – 372. Bowie, A.R., Maldonado, M.T., Frew, R.D., Croot, P.L., Achterberg, E.P., Mantoura, R.F.C., Worsfold, P.J., Law, C.S., Boyd, P.W., 2001. The fate of added iron during a mesoscale fertilisation experiment in the Southern Ocean. Deep-Sea Research: Part II. Topical Studies in Oceanography 48, 2703 – 2743. Boyd, P., LaRoche, J., Gall, M., Frew, R., McKay, R.M.L., 1999. Role of iron, light, and silicate in controlling algal biomass in subantarctic waters SE of New Zealand. Journal of Geophysical Research—Oceans 104, 13395 – 13408. Boyd, P.W., Watson, A.J., Law, C.S., Abraham, E.R., Trull, T., Murdoch, R., Bakker, D.C.E., Bowie, A.R., Buesseler, K.O., Chang, H., Charette, M., Croot, P., Downing, K., Frew, R., Gall, M., Hadfield, M., Hall, J., Harvey, M., Jameson, G., LaRoche, J., Liddicoat, M., Ling, R., Maldonado, M.T., McKay, R.M., Nodder, S., Pickmere, S., Pridmore, R., Rintoul, S., Safi, K., Sutton, P., Strzepek, R., Tanneberger, K., Turner, S., Waite, A., Zeldis, J., 2000. A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization. Nature 407, 695 – 702. Boyle, E., 1998. Oceanography—pumping iron makes thinner diatoms. Nature 393, 733 – 734. Bradford-Grieve, J.M., Chang, F.H., Gall, M., Pickmere, S., Richards, F., 1997. Size-fractionated phytoplankton standing stocks and primary production during austral winter and spring 1993 in the Subtropical Convergence region near New Zealand.

158

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159

New Zealand Journal of Marine and Freshwater Research 31, 201 – 224. Brzezinski, M.A., Conley, D.J., 1994. Silicon deposition during the cell cycle of Thalassiosira weissflogii (Bacillariophyceae) determined using dual rhodamine 123 and propidium iodide staining. Journal of Phycology 30, 45 – 55. Busey, R.H., Mesmer, R.E., 1977. Ionisation equilibria of silicic acid and polysilicate formation in aqueous sodium chloride solutions to 300 jC. Inorganic Chemistry 16, 2444 – 2450. Darby, W.M., Volcani, B.E., 1969. Role of silicon in diatom metabolism. A silicon requirement for deoxyribonucleic acid synthesis in the diatom Cylindrotheca fusiformis Reimann and Lewin. Experimental Cell Research 58, 334 – 343. de Baar, H.J.W., Dejong, J.T.M., Bakker, D.C.E., Loscher, B.M., Veth, C., Bathmann, U., Smetacek, V., 1995. Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean. Nature 373, 412 – 415. Del Amo, Y., Brzezinski, M.A., 1999. The chemical form of dissolved Si taken up by marine diatoms. Journal of Phycology 35, 1162 – 1170. De la Rocha, C.L., Brzezinski, M.A., DeNiro, M.J., 1997. Fractionation of silicon isotopes by marine diatoms during biogenic silica formation. Geochimica et Cosmochimica Acta 61, 5051 – 5056. De la Rocha, C.L., Brzezinski, M.A., DeNiro, M.J., 2000. A first look at the distribution of the stable isotopes of silicon in natural waters. Geochimica et Cosmochimica Acta 64, 2467 – 2477. Downes, M.T., 1978. An automated determination of low reactive phosphorus concentrations in natural waters in the presence of arsenic, silicon, and mercuric chloride. Water Research 12, 743 – 745. Duce, R.A., Liss, P.S., Merrill, J.T., Atlas, E.L., Buat-Menard, P., Hicks, B.B., Miller, J.M., Prospero, J.M., Arimoto, R., Church, T.M., Ellis, W., Galloway, J.N., Hansen, L., Jickells, T.D., Knap, A.H., Reinhardt, K.H., Schneider, B., Soudine, A., Tokos, J.J., Tsunogai, S., Wollast, R., Zhou, M., 1991. The atmospheric input of trace species to the world ocean. Global Biogeochemical Cycles 5, 193 – 259. Elderfield, H., Rickaby, R.E.M., 2000. Oceanic Cd/P ratio and nutrient utilization in the glacial Southern Ocean. Nature 405, 305 – 310. Ellwood, M.J., Maher, W.A., 2002. An automated hydride generation – cryogenic trapping – ICP – MS system for measuring inorganic and methylated Ge, Sb and As species in marine and fresh waters. Journal of Analytical Atomic Spectrometry 17, 197 – 203. Froelich, P.N., Andreae, M.O., 1981. The marine geochemistry of germanium—Ekasilicon. Science 213, 205 – 207. Froelich, P.N., Hambrick, G.A., Andreae, M.O., Mortlock, R.A., Edmond, J.M., 1985. The geochemistry of inorganic germanium in natural waters. Journal of Geophysical Research—Oceans 90, 1133 – 1141. Froelich, P.N., Mortlock, R.A., Shemesh, A., 1989. Inorganic germanium and silica in the Indian ocean: biological fractionation during (Ge/Si)opal formation. Global Biogeochemical Cycles 3, 79 – 88. Froelich, P.N., Vlanc, V.R., Mortlock, R.A., Chillaud, S.N., Dustan,

W., Udomkit, A., Peng, T.H., 1992. River fluxes of dissolved silica to the ocean were higher during glacials: Ge/Si in diatoms, rivers and oceans. Paleoceanography 7, 739 – 767. Hammond, D.E., McManus, J., Berelson, W.M., Meredith, C., Klinkhammer, G.P., Coale, K.H., 2000. Diagenetic fractionation of Ge and Si in reducing sediments: the missing Ge sink and a possible mechanism to cause glacial/interglacial variations in oceanic Ge/Si. Geochimica et Cosmochimica Acta 64, 2453 – 2465. Hildebrand, M., Volcani, B.E., Gassmann, W., Schroeder, J.I., 1997. A gene family of silicon transporters. Nature 385, 688 – 689. Hildebrand, M., Dahlin, K., Volcani, B.E., 1998. Characterization of a silicon transporter gene family in Cylindrotheca fusiformis: sequences, expression analysis, and identification of homologs in other diatoms. Molecular and General Genetics 260, 480 – 486. Hutchins, D.A., Bruland, K.W., 1998. Iron-limited diatom growth and Si:N uptake ratios in a coastal upwelling regime. Nature 393, 561 – 564. Jin, K., Shibata, Y., Morita, M., 1991. Determination of germanium species by hydride generation inductively coupled argon plasma mass spectrometry. Analytical Chemistry 63, 986 – 989. King, S.L., Froelich, P.N., Jahnke, R.A., 2000. Early diagenesis of germanium in sediments of the Antarctic South Atlantic: in search of the missing Ge sink. Geochimica et Cosmochimica Acta 64, 1375 – 1390. Kumar, N., Anderson, R.F., Mortlock, R.A., Froelich, P.N., Kubik, P., Dittrichhannen, B., Suter, M., 1995. Increased biological productivity and export production in the glacial Southern Ocean. Nature 378, 675 – 680. Lewin, J.C., 1955. Silicon metabolism in diatoms: III. Respiration and silicon uptake in Navicula pelliculosa. Journal of General Physiology 39, 1 – 10. Lewis, B.L., Froelich, P.N., Andreae, M.O., 1985. Methylgermanium in natural waters. Nature 313, 303 – 305. Lewis, B.L., Andreae, M.O., Froelich, P.N., Mortlock, R.A., 1988. A review of the biogeochemistry of germanium in natural waters. Science of the Total Environment 73, 107 – 120. Lewis, B.L., Andreae, M.O., Froelich, P.N., 1989. Sources and sinks of methylgermanium in natural waters. Marine Chemistry 27, 179 – 200. Martin, J.H., Gordon, R.M., Fitzwater, S.E., 1990. Iron in Antarctic waters. Nature 345, 156 – 158. Mehard, C.W., Sullivan, C.W., Azam, F., Volcani, B.E., 1974. Role of silicon in diatom metabolism: IV. Subcellular localization of silicon and germanium in Nitzschia alba and Cylindrotheca fusiformis. Physiologia Plantarum 30, 265 – 272. Mortlock, R.A., Froelich, P.N., 1987. Continental weathering of germanium—Ge/Si in the global river discharge. Geochimica et Cosmochimica Acta 51, 2075 – 2082. Mortlock, R.A., Froelich, P.N., 1996. Determination of germanium by isotope dilution hydride generation inductively coupled plasma mass spectrometry. Analytica Chimica Acta 332, 277 – 284. Mortlock, R.A., Charles, C.D., Froelich, P.N., Zibello, M.A., Saltzman, J., Hays, J.D., Burckle, L.H., 1991. Evidence for lower productivity in the Antarctic Ocean during the last glaciation. Nature 351, 220 – 223.

M.J. Ellwood, W.A. Maher / Marine Chemistry 80 (2003) 145–159 Murnane, R.J., Stallard, R.F., 1988. Germanium/silicon fractionation during biogenic opal formation. Paleoceanography 3, 461 – 469. Murnane, R.J., Leslie, B., Hammond, D.E., Stallard, R.F., 1989. Germanium Geochemistry in the Southern-California Borderlands. Geochimica et Cosmochimica Acta 53, 2873 – 2882. Nydahl, F., 1976. On the optimum conditions for the reduction of nitrate to nitrite by cadmium. Talanta 23, 349 – 357. Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., Pepin, L., Ritz, C., Saltzman, E., Stievenard, M., 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399, 429 – 436. Pokrovski, G.S., Schott, J., 1998. Thermodynamic properties of aqueous Ge(IV) hydroxide complexes from 25 to 350 degrees C: implications for the behavior of germanium and the Ge/Si ratio in hydrothermal fluids. Geochimica et Cosmochimica Acta 62, 1631 – 1642. Ragueneau, O., Treguer, P., Leynaert, A., Anderson, R.F., Brzezinski, M.A., DeMaster, D.J., Dugdale, R.C., Dymond, J., Fischer, G., Francois, R., Heinze, C., Maier-Reimer, E., Martin-Jezequel, V., Nelson, D.M., Queguiner, B., 2000. A review of the Si cycle in the modem ocean: recent progress and missing gaps in the application of biogenic opal as a paleoproductivity proxy. Global and Planetary Change 26, 317 – 365.

159

Santosa, S.J., Wada, S., Mokudai, H., Tanaka, S., 1997. The contrasting behaviour of arsenic and germanium species in seawater. Applied Organometallic Chemistry 11, 403 – 414. Shemesh, A., Mortlock, R.A., Froelich, P.N., 1989. Late Cenozoic Ge/Si record of marine biogenic opal: implications for variations of riverine fluxes to the ocean. Paleoceanography 3, 221 – 234. Smith, J.D., Milne, P.J., 1981. Spectrophotometric determination of silicate in natural waters by formation of a-molybdosilicic acid and reduction with a tin(IV) – ascorbic acid – oxalic mixture. Analytica Chimica Acta 123, 263 – 270. Strickland, J.D.H., Parsons, T.R., 1972. A Practical Handbook of Seawater Analysis. Bulletin of the Fisheries Research Board of Canada, Ottawa, Canada, vol. 167. 311 pp. Sullivan, C.W., 1976. Diatom mineralization of silicic acid: I. Si(OH)4 transport characteristics in Navicula pelliculosa. Journal of Phycology 12, 390 – 396. Sullivan, C.W., Volcani, B.E., 1973. Role of silicon in diatom metabolism: 3. The effects of silicic acid on DNA polymerase, TMP kinase amd DNA synthesis in Cylindrotheca fusiformis. Biochimica et Biophysica Acta 308, 212 – 229. Takeda, S., 1998. Influence of iron availability on nutrient consumption ratio of diatoms in oceanic waters. Nature 393, 774 – 777. Wu, J.F., Sunda, W., Boyle, E.A., Karl, D.M., 2000. Phosphate depletion in the western North Atlantic Ocean. Science 289, 759 – 762.