From diatom opal-A δ18O to chert δ18O in deep sea sediments

Journal Pre-proofs From diatom opal-A δ 18O to chert δ 18O in deep sea sediments A.G. Yanchilina, R. Yam, Y. Kolodny, A. Shemesh PII: DOI: Reference:

S0016-7037(19)30662-3 https://doi.org/10.1016/j.gca.2019.10.018 GCA 11486

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Geochimica et Cosmochimica Acta

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14 October 2018 10 October 2019 11 October 2019

Please cite this article as: Yanchilina, A.G., Yam, R., Kolodny, Y., Shemesh, A., From diatom opal-A δ 18O to chert δ 18O in deep sea sediments, Geochimica et Cosmochimica Acta (2019), doi: https://doi.org/10.1016/j.gca. 2019.10.018

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From diatom opal-A δ18O to chert δ18O in deep sea sediments A.G. Yanchilinaa*, R. Yama, Y. Kolodnyb, A. Shemesha aDepartment

of Earth and Planetary Sciences, Weizmann Institute of Science, 234 Hertzl Street, Rehovot, Israel

7610001. bInstitute

of Earth Sciences, the Hebrew University of Jerusalem, The Edmond J. Safra Campus – Givat Ram,

Jerusalem, Israel 9190401. * Author to whom correspondence should be addressed; email: [email protected] (A.G. Yanchilina)

Abstract δ18O signature of marine deep-sea cherts was previously used to reconstruct past ocean temperature and oceanic bottom water δ18O through the Cenozoic and Mesozoic periods. Oxygen isotopes in deep-sea cherts which were never exposed to meteoric water exhibit a wide range of values indicating that the evolution and maturation of biogenic amorphous opal (opalA) to opal-CT and microquartz chert is accompanied by isotopic changes. We measured δ18O of diatom opal-A, opal-CT, and microquartz chert in downcore profiles of sediment cores ODP 795 and 799 from the Sea of Japan. Diatom opal-A, opal-CT and microquartz chert were separated and purified from the bulk sediment. This was followed by XRD, SEM/EDS and XPS to identify and quantify the mineral phases, the qualitative elemental concentrations and the amounts of authigenic clays. Diatom opal-A transforms to opal-CT and microquartz chert when it is deposited in sediment originally rich in diatoms relative to other lithologic constituents, such as clay. Diatom opal-A δ18O is in the range of 34-38 ‰. δ18O of opal-CT is ~25 ‰ at the first phase transformation at 330 mbsf in ODP 795 and 420 mbsf in ODP 799. This phase transformation occurs at a temperature of about 40 ºC. δ18O of opal-CT remains essentially unchanged until it undergoes the second phase transformation to microquartz chert at 470 mbsf in ODP 795 and at 580 mbsf in ODP 799, at which point its δ18O decreases abruptly to ~20 ‰. This phase

transformation occurs at a temperature of about 60 ºC. The δ18O values of opal-CT and microquartz chert appear to reflect equilibrium formation temperatures of silica corresponding to the geothermal gradient and the local porewater δ18O. The δ18O of opal-CT and microquartz chert are controlled by the geothermal temperatures and compositions of pore waters during polymorphic transformations deep within the sediment, indicating that the δ18O of these phases cannot be used to determine temperature or composition of seawater during diatom growth.

1. INTRODUCTION Chert, a sedimentary rock composed of authigenic microcrystalline quartz is an abundant and conspicuous rock throughout the geologic record, from the Archean to the Late Cenozoic (Keene and Kastner, 1974; Murray et al., 1992). As ubiquitous as these rocks are, their origin, though discussed for more than a century has not been uniquely solved (Marin-Carbonne et al., 2012; Marin-Carbonne et al., 2014; Tartèse et al., 2017). δ18O of Precambrian cherts (before 550 million years ago) have been used to estimate the temperature of the Precambrian ocean, a parameter that still has not been constrained with temperature estimates varying between as high as 85 C (Knauth and Lowe, 1978; Knauth and Lowe, 2003) to as low as 37-52 C (Marin et al., 2010) and  40 C (Hren et al., 2009). A significant contribution to the problem has recently been made by Galili et al. (2019) who showed that the δ18O of seawater was different in the Archean relative to the Phanerozoic. Robert and Chaussidon (2006) showed, using both δ18O and δ30Si of Precambrian cherts, that ocean temperature decreased from 70C to about 20C between 3500 to 800 million years ago. δ18O of deep sea cherts has been used to reconstruct oceanic bottom temperatures over the past 150 Myr, assuming that it is controlled by bottom seawater temperature and the δ18O of the seawater (Knauth and Epstein, 1975; Kolodny and Epstein, 1976). A traditionally agreed upon sequence of transformation of biogenic opal as a consequence of diagenesis has been opal-A → opal-CT → microquartz chert.

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Opal-A is an optically isotropic, cryptocrystalline, hydrous and disordered form of silica (Jones and Segnit, 1971). Opal-CT is disordered interlayering of α-cristobalite and α-tridymite (Jones and Segnit, 1971; Kastner et al., 1977). Chert is composed of microcrystalline α-quartz (Knauth, 1994). We will refer hereafter to the micro-crystalline form of chert as microquartz chert. As of the Phanerozoic, opal-A is largely produced by silicification by radiolarians, and as of the Jurassic, primarily by diatoms. This process occurs only in the uppermost layer of the ocean because of the light requirements for photosynthesis. δ18O of diatoms has been used to reconstruct past surface ocean δ18O and surface temperatures (Abelmann et al., 2015; Leng and Barker, 2006; Shemesh et al., 1992). It was suggested that δ18O of biogenic opal-A may instead reflect bottom water / surface sediment porewater δ18O and temperature as a consequence of dihydroxylation/ condensation combined with reprecipitated dissolved silica upon burial (Dodd et al., 2012; Dodd et al., 2017; Schmidt et al., 1997). However, the exact fraction of the original biogenic opal-A retained upon accumulation in sediment has yet to be fully studied. After burial, biogenic opal-A dissolves and reprecipitates as metastable opal-CT and microquartz chert (Kastner et al., 1977; Murata and Nakata, 1974; Wise et al., 1972). Hein et al. (1978) showed that after sufficient burial, frustules are fragmented and undergo mild dissolution, after which silica reprecipitates as inorganic opal-A’ and only then becomes rapidly transformed to opal-CT by crystal growth. Opal-A’ is rarely observed and exists over only a few meters in the sediment. Murata et al. (1977) proposed that opal-CT also dissolves again prior to reprecipitating into a more thermodynamically stable form of microquartz chert. Ernst and Calvert (1969) and Heath and Moberly (1971), on the other hand, proposed that microquartz chert crystallizes directly from opal-CT without an intermediate solutionprecipitation step. Hence, formation of deep-sea cherts has remained an open question.

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The δ18O of diatom opal-A is controlled by water temperature and the δ18O of the water from which the frustule formed (Juillet-Leclerc and Labeyrie, 1987; Leng and Barker, 2006; Shemesh et al., 1992). It has been previously observed that there is a difference in δ18O of the two diagenetic phases with that of microquartz chert being lower δ18O value than opal-CT (Kolodny and Epstein, 1976; Matheney and Knauth, 1993). The difference has been attributed to a difference in temperature of formation of the two phases (Matheney and Knauth, 1993). Sharp et al. (2016) note that fractionation between water and silica, for both biogenic opal-A and high temperature microquartz chert, are nearly identical and the difference between the two fractionations can be treated as negligible. Heath and Moberly (1971) speculate that only at temperatures of about 30-50° will microquartz chert become the dominant silica mineral. Murata et al. (1977) show that calculated temperatures of formation from the δ18O of opal-CT and microquartz chert are in the range of 48°C and 79°C, respectively. They interpret that the transformation is primarily a process of solution-reprecipitation during which the newly formed phase acquires a δ18O that reflects the ambient temperature and δ18O of porewater. Matheney and Knauth (1993) show that for the Monterey Formation in California, δ18O of opal-CT indicates it formed at lower temperatures of 17-21°C after taking into account that the porewater in which the opal-CT formed had a lower δ18O. δ18O of porewater in the sediment can be altered by formation of microquartz chert and clays from other materials such as igneous sources that have relatively low δ18O values (Brumsack et al., 1992). Alteration of volcanic material decreases the δ18O of porewater (Brumsack et al., 1992; Lawrence et al., 1975) while alteration of opal-CT and microquartz chert increase the δ18O of porewater (Murata et al., 1977). Lancelot (1973) stressed that it is not only the temperature that is responsible for the phase transformations of silica but also, the lithology of the host sediment. Whether the sediment is clay or carbonate may be a factor that controls the evolution of silica. Kastner et al. (1977) likewise, suggested that porewater alkalinity, a parameter linearly correlated with

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the concentration of magnesium in the porewater, also influences the temperature of the silica phase transitions. In experiments with higher concentrations of magnesium and hydroxyl in the porewater, formation of opal-CT is enhanced. In clay dominant environments, opal-A to opal-CT transition initiates later and at deeper depths whereas opal-CT to microquartz chert transition starts earlier and at shallower depths (Keene and Kastner, 1974; Williams and Crerar, 1985; Williams and Parks, 1984). In order to reconstruct past bottom ocean temperature and seawater isotopic composition through time, the remaining questions involve tracing the changes in the initial δ18O of opal-A to opal-CT and microquartz chert as a consequence of diagenesis. While a number of studies have focused on the interpretation of the δ18O of opal-CT and microquartz chert in deep-sea sediments, none have explored the full oxygen isotope path of formation of microquartz chert from biogenic opal-A. We use the recent developments in the oxygen isotope measurements of deep-sea biogenic siliceous sediments to fully follow the process of microquartz chert formation in two ODP cores, 795 and 799, where these three phases of silica occur in consecutive order in the same sediment cores and for which geothermal temperatures and pore water δ18O have been measured. We aim to determine the relation between the phase transition, the silica δ18O and the environmental factors of temperature and porewater δ18O. 2. MATERIALS AND METHODS Samples from Ocean Drilling Project (ODP) cores 795 and 799 were taken from each of the sections that were identified to have opal-A, opal-CT, and chert lithologies. Sample depths were selected on the basis of the initial ODP proceedings (Ingle et al., 1990; Tamaki et al., 1990) which described the transition depths from biogenic opal-A to opal-CT and opal-CT to microquartz chert. These cores were retrieved on ODP legs 127 and 128 in the Sea of Japan (Fig. 1) at water depths of 3311 and 2073 meters. ODP 795 is more susceptible to terrigenous and volcanic input with several volcanic layers noted in the core lithologies (Tamaki et al.,

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1990). Local temperature was measured using the Uyeda probe at 5 horizons down to 162.4 mbsf in ODP hole 795A with a calculated temperature gradient of 133ºC/km (Tamaki et al., 1990) and at 8 horizons down to 174.5 mbsf in ODP hole 799A with a calculated temperature gradient of 98ºC/km (Ingle et al., 1990). Opal-A to opal-CT transition occurs at 330 mbsf with a burial temperature of 43ºC in ODP 795 (Tamaki et al., 1990) and at 420 mbsf with a burial temperature of 40ºC in ODP 799 (Ingle et al., 1990). Opal-CT to chert transition occurs at 470 mbsf with a burial temperature of 63ºC in ODP 795 (Tamaki et al., 1990) and at 585 mbsf with a burial temperature of 57ºC in ODP 799 (Ingle et al., 1990). Age models for the cores were determined from microfossil chronostatigraphy (Ingle et al., 1990; Tamaki et al., 1990). ODP 795 spans the past 14 Myr while ODP 799 covers the past 23 Myr. The age model for ODP 799 for core depths >420 mbsf is only approximate as preservation of microfossils is poor and therefore, correct identification for chronostatigraphic reconstruction becomes less accurate. We labeled it as Early, Middle, and Late Miocene in accordance with the ODP report (Ingle et al., 1990). Samples were first crushed with mortar and pestle and were then cleaned according to the procedure developed for isolating the diatom fraction out of bulk sediment as described in Shemesh et al. (1992) and Crespin et al. (2014). After initial sieving via sieves of 32-63 µm, 63-125 µm, and 125-250 µm we find that the 32-63 µm size fraction contained the majority of diatom opal-A. Altered diatoms also were separated in this size fraction as were some of recrystallized diatoms to the opal-CT phase. In the deeper sections of the cores, no material in the size fraction of >32 µm was observed, as all silica was in the opal-CT and/or microquartz chert phase composed the smaller size fraction of the bulk sediment and no original diatom structure was observed. Following the separation of opal-A and opal-CT, the chert containing samples were treated with 1:5 HCl solution until no reaction between the acid and the carbonates was observed (Kolodny and Epstein (1976). Diatom opal-A was separated from

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bulk sediment by a series of centrifugations and SPT density separations. Opal-CT was separated from microquartz chert by density in sodium polytungstate solutions (SPT). Although diatom opal-A has a density of 2.1 g/cm3 and opal-CT has an overlapping density of 1.9-2.3 g/cm3 the two phases did not co-exist in our samples (Eckert, 1997; Mustoe, 2016). The efficiency of the separation was routinely checked with Petrographic Microscopy (PM) smear slides followed by SEM/EDS, XRD, and XPS. After cleaning, all samples were dried in an oven at 40ºC. Mineralogical phases were identified at the Weizmann Institute of Science by X-ray diffraction (XRD) on the Sealed Cu-anode tube (line source) Rigaku ULTIMA III (2kW) thetatheta vertical diffractometer equipped with CBO and Bruker D2 Phaser, using Cu Kα radiation. Both instruments were used interchangeably, depending on availability. XRD patterns were automatically compared with an International Center for Diffraction Data (ICDD) database with Jade software package. On XRD spectra, opal-A is recognized by a broad diffuse peak centered at 4.1 Å (22.2° in 2θ Cu K-alpha) (Jones and Segnit, 1971). This diffuse peak spreads from 17° to 30° in diatom opal-A, reflecting the amorphous and highly disordered nature of biogenic opal secreted by silica microfossils when constructing their skeletons. Opal-CT is identified by broad peaks centered at

4.11 Å and 2.50 Å (Jones and Segnit, 1971),

corresponding to 20-24° centered on 22.2° and 36-37° as a function of 2θ Cu K-alpha. The 4.11 Å peak reflects the 101 plane and the 2.50 Å peak reflects the 200 plane of α-cristobalite (Ernst and Calvert, 1969; Wilson, 2014). Opal-CT is traditionally described to consist of disordered α-cristobalite and α-tridymite (Jones and Segnit, 1971). Increase in the ordering and a larger proportion of α-cristobalite relative to α-tridymite is reflected as the d spacing of the diffraction maximum at 4.11 Å approaches 4.04 Å of α-cristobalite (Wilson, 2014). Weaker reflection at 4.25 Å, 3.33 Å, and 1.81 Å correspond to the 1010, 1011, and 1122 of α-quartz (Ernst and Calvert, 1969). Microquartz chert is identified by two main distinct peaks at 4.26

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Å (20.8° in 2θ Cu K-alpha) and at 3.33 Å (26.7° in 2θ Cu K-alpha) of α-quartz (Ernst and Calvert, 1969; Meister et al., 2014; Mizutani, 1977). These two peaks correspond to the 1010 and 1011 planes of α-quartz. We use two parameters from XRD spectra to evaluate maturation and diagenesis of biogenic opal-A to opal-CT and microquartz chert. The first parameter is the ratio of microquartz 1011 to α-cristobalite 101 peak heights to diagnose the amount of quartz relative to opal-CT, a ratio which reflects the amount of recrystallization that took place (Ernst and Calvert, 1969). The second parameter is the full width at half maximum (FWHM) of the α-cristobalite 101 peak which indicates the degree of crystallization of opal-A to microquartz chert. This parameter shows the degree of mineralogical maturation, from noncrystalline opalA and opal-A/CT to paracrystalline opal-CT and opal-C and ultimately to microquartz chert (Perry et al., 2006). Mineralogically mature samples with increased order of silica structure are characterized by lower FWHM values and sharper peaks (e.g. Lynne et al., 2005). The 101 peak of opal-CT decreases significantly upon full transition from opal-A to microquartz chert, while the 1010 and 1011 peaks of α-quartz increase. The width of the peaks of opal-CT (their FWHM) also decrease significantly, as the more distinct and thin peaks belonging to α-quartz appear. Selected samples, after cleaning and purification, were also inspected with the Scanning Electron Microscope with Energy Dispersive Spectroscopy (SEM/EDS) on the Zeiss Supra 55. We use a ratio of aluminum relative to silica retrieved from the SEM/EDS spectra to identify the alteration / reaction with clays and to qualitatively document the presence of aluminum silicates. Voltage was set to 13 keV with a 30 µm aperture size. The size of the captured image was 1000 pixels with a 300 s exposure time. These samples were also inspected with the Xray photoelectron spectroscopy (XPS) on the Kartos X-ray Photoelectron Spectrometer – Axis Ultra DLD equipped with a monochromatic Al Kα source (1486.6 eV) at 75 W. Detection pass energies ranged between 20 and 80 eV. Low-energy electron flood gun was applied for charge

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neutralization. For depth profiling a Kratos Argon Ion Source (Ar+ GS) was applied. This procedure allowed the determination of elemental distribution on the surface of silica samples and beneath. We applied 1 and 3 min. sputtering to measure the elemental distribution beneath the surface at two different depths. These sputtering times theoretically correspond to ablation depths of 6 nm and 18 nm, respectively (the ablation speed being 0.1 nm per second). Samples were sputtered with source high tension (HT) of 3.8 keV and extractor current of 50 A. XPS results are reported as a mass concentration of each element. Silica identified as diatom opal-A or opal-CT was then subject to controlled isotope exchange with heavy water with a δ18O of 34.7 ‰ followed by high temperature recrystallization at 980ºC (Crespin et al., 2014; Juillet-Leclerc and Labeyrie, 1987; Shemesh et al., 1995). δ18O of silica was measured with an Infra-Red (IR) laser extraction technique with a Merchanteck 25W CO2 IR laser using BrF5 as the reagent to extract the molecular O2 from 200 µg of the silica (Crespin et al., 2014). δ18O of O2 was measured with a Thermo Finnigan Delta Plus XL isotope-ratio mass spectrometer in continuous flow mode and calibrated with NBS-28 and the PS diatom standards (Chapligin et al., 2011). Results are reported relative to V-SMOW. Both NBS-28 and QSN were introduced in each batch of fluorination to maintain consistency over time while the PS standard was run periodically. Average measured δ18O of the NBS-28 was -13.137 ‰ with a standard deviation of 0.435 ‰ and average measured δ18O of the QSN was -6.004 ‰ with a standard deviation of 0.410 ‰. Silica identified as microquartz chert was not subject to any isotope exchange as prior measurements by Knauth and Epstein (1976) indicated that chert does not have any exchangeable oxygen. Overall, 69 samples were measured from the two cores. 58 samples were run at least in triplicates. 8 samples were run in duplicates and 1 with no replication due to insufficient amounts. 3. RESULTS

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We have identified four phases of silica after purification from the bulk sediment: diatom opal-A, “altered” diatom opal-A, opal CT and microquartz chert (Table 1 and 2; Fig. 2). Above the opal-A to opal-CT transition zones in ODP 795 and 799 two types of amorphous silica exist, diatoms consisting of pure opal-A and diatoms containing aluminum silicates but maintain the original morphological characteristics. We refer to those as “altered” diatom opalA. Phases were identified according to their XRD spectra while the purity of the samples according to results from SEM/EDS and XPS. Pure diatom opal-A becomes abundant below 180 mbsf in ODP 795 whereas “altered” diatom opal-A is abundant and dominant above that depth (and above respective opal-A/ opal-CT transition zones in the two cores). In ODP 799, pure diatom opal-A becomes abundant below 200 mbsf whereas “altered” diatom opal-A is dominant above 200 mbsf. Best-preserved diatoms correspond to high opal content in the sediment relative to clay. Samples that were defined as pure diatom opal-A by XRD (Table 1 and 2; Fig. 3A; Fig. 7A and B) present low aluminum and iron relative to silica (SEM/EDS and XPS results in Fig. 3 and Al/Si in 7C). XPS results show that aluminum concentration varies around 1.45% and iron concentration around 0.65%. This composition is consistent on the surface of the diatoms and in the inner structure. For all of the samples characterized as diatom opal-A, the peak height ratio of the 26.7° to 22.2° peaks ranges between 0.6 and 2 (Fig. 7A), the FWHM of the 101 peak ranges between 7 and 8.5 (FIG. 7B), and the Al:Si ranges between 0 to 0.1 (FIG. 7C). Detrital quartz, recognized by a peak at 26.7 ° in 2θ Cu K-alpha, is not observed. “Altered” diatom opal-A looks very different in XRD spectra. The smooth diffuse peak characteristic of diatom opal-A is highly suppressed (Fig. 4A). The peak characteristic of quartz, at 26.7°, is distinct and hence the peak ratio of 26.7° to 22.2° elevated. FWHM is about 2.4°, smaller than the 8° of opal-A. Additional peaks at ~19° ~28° are associated with

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aluminosilicates. SEM/EDS indicate aluminum, iron, potassium, sodium, and magnesium attached to the surface (Fig. 4C-F). XPS indicates that the altered chemical composition is not unique to the surface; the aluminum and iron have become part of the original inner structure of the diatom (Fig. 4B). For all of the samples characterized as “altered” diatom opal-A, the peak height ratio of the 26.7° to 22.2° peaks ranges between 1.5 and 7.5 (Fig. 7A), the FWHM of the 22.2° peak ranges between 0.1 and 12 (FIG. 7B), and the Al:Si ranges between 0.1 to 0.35 (FIG. 7C). Opal-CT is measured at 300 to 500 mbsf in ODP 795 and 500 to 600 mbsf in ODP 799 (Fig. 2). This phase is observed to co-exist with more chertified silica at 430 to 500 mbsf in ODP 795 and at 500 to 600 mbsf in ODP 799 (Fig. 2). Opal-CT was recognized by XRD, using the two strong and distinct peaks at 20-24° and 36-37° (Fig. 5A and 5E). The samples show different levels of maturation according to their peak ratios and FWHM, where opal-CT phase is more expressed (Figs. 5 A-D) and microquartz chert becomes the dominant phase (Figs. 5 E-H). SEM/EDS and XPS measurements show that the amount of Fe and Al in the matrices of the two samples are similar (Fig. 5 B, D, F, H). XPS indicates 4.5 % of Al and 1.55 % of Fe for the opal-CT. For microquartz chert Al content is 5.4% and Fe is 1.8 %. SEM/EDS shows that Al:Si is 0.10 for the opal-CT and 0.15 for the microquartz chert. Fe concentrations are not significant. For all of the samples characterized as opal-CT, the peak height ratio of the 26.7° to 22.2° peaks ranges between 0.7 and 5 (Fig. 7A), the FWHM of the 22.2° peak ranges between 0.5 and 6 (FIG. 7B), and the Al:Si ranges between 0.1 to 0.2 (FIG. 7C). Pure microquartz chert becomes dominant in the sediment below 500 mbsf in ODP 795 and below 620 mbsf in ODP 799 (Fig. 2). In XRD, the 20.8° peak is small whereas the 26.7° peak is much stronger; both are distinct (Fig. 6A). In the case where all of the opal-CT recrystallized to microquartz chert, peaks associated with opal-CT, 22.2° and 36-37°, are highly suppressed or absent. The ratio of the two peaks in pure microquartz chert is ~33 and

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the FWHM is ~0.1. SEM/EDS indicate Al:Si content of 0.11 (Fig. 6C-D) and XPS indicate aluminum concentration of 6.3 % and iron of 2.15 % (Fig. 6 B). The peak height ratio of the 26.7° to 22.2° peaks ranges between 3 and 50 (Fig. 7A), the FWHM of the 22.2° peak ranges from 0.1 to 0.5 (FIG. 7B), and the Al:Si ranges between 0.09 to 0.12 (FIG. 7C). δ18O of pure opal-A diatom is high (34-38 ‰) and δ18O of “altered” diatom opal-A is lower (generally in the range of 20 to 33 ‰) (Fig. 2; Fig. 7D). The highest δ18O of pure diatom opal-A is 37.6 ± 0.6 ‰ with an age of 4.32 Myr and at a depth of 319 mbsf from ODP 799. The δ18O of “altered” diatoms is lowest at the top of both cores and increases with depth from values of 18 ‰ to 33 ‰. In both cores opal-CT and microquartz chert occur below the opal-A phases. Opal-CT is characterized by a δ18O of ~< 25 ‰, and microquartz chert is characterized by δ18O of about 20 ‰ (Fig. 1). The δ18O values of opal-CT range from 20 to 28 ‰ and the δ18O values of microquartz chert range from 18 to 22 ‰ (Fig. 7D).

DISCUSSION The measured δ18O of diatom opal-A for both cores, which has had minimal reaction with clay material and formation of authigenic clays, is 34-38 ‰ (Fig. 3). According to both SEM/EDS and XPS (Figs. 3C, 3D, 3B), these samples have very small amounts of elements other than Si and O and represent the purest amorphous silica. These δ18O values of diatom opal-A are similar to modern marine diatoms and are interpreted to preserve an original surface ocean δ18O signal (Juillet-Leclerc and Labeyrie, 1987; Leng and Barker, 2006; Shemesh et al., 1992) or with some degree of alteration at bottom water/surface sediment interface (Dodd et al., 2012; Dodd et al., 2017; Schmidt et al., 1997). Either way, once dihydroxylation has taken place, diatom opal-A δ18O is stable until the phase changes occur in the sediment (Dodd et al., 2017; Juillet-Leclerc and Labeyrie, 1987; Labeyrie and Juillet, 1982; Murata et al., 1977).

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At the top of ODP 795, δ18O of “altered” diatom opal-A varies around 20 ‰, ranging between 18 and 23 ‰ from the top of the core to 135 mbsf (Fig. 2A). At the top of ODP 799, δ18O of “altered” diatom opal-A also varies around 20 ‰ and in the range of 20 to 33 ‰ (Fig. 2B). SEM/EDS and XRD have identified this opal to contain aluminum and iron oxides as integral part of the diatom structure (Fig. 4). These are diatoms that have had authigenic clays begin replacing the original silica frustule as a diagenetic process (Figs. 4C-F). We interpret this high iron - aluminum content as authigenic mineral growth replacing the diatom opal-A frustules (and/or other components of opal-A such as radiolarian tests and sponge spicules). This process is reported to occur in high clay environments where the diatoms and other microfossil opal-A skeletons become transformed to clay (Michalopoulos and Aller, 2004; Michalopoulos et al., 2010). The amount of aluminum of this opal relative to pure diatoms (illustrated as an Al/Si ration from SEM/EDS) is elevated (i.e., 0.2-0.4 relative to <0.15) (Figs. 4B-F, Fig. 7C). We note that there is no precise element calibration in both, SEM/EDS and XPS. XRD spectra of the isolated maximally “altered” diatom opal-A show that although diatom frustule structure is preserved (Figs. 4A), the intensity of the diffuse peak of opal-A centered on 22.2° (101) is suppressed relative to the intensity of quartz at 26.7° (1011) (Fig. 4A). Hein et al. (1978) likewise noted that a broad peak of amorphous opal decreases in amplitude and broadens with significant amounts of clay. The XRD diffuse peak at 19° is attributed to illite, smectite, and kaolinite and at 27° is attributed to feldspar; these authigenic minerals are noted to diagenetically alter diatom structure in environments of high clay content (Balderman and Warr, 2013; Hein et al., 1978; van Bennekom et al., 1989). Peaks at 35° and 61-62° are also likely an indication of amorphous alumino-silicate formation. We interpret that all of “altered” diatoms in both cores that initially accumulated in sediments of high clay relative to diatom opal-A are altered and en route to become clays (Fig. 2). Lowest δ18O of “altered” diatoms occur at the top of both cores, where clay to opal diatom-

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A is higher relative to the sections of 200 to 320 mbsf in ODP 795 and 200 to 430 mbsf in ODP 799. As δ18O of typical marine clays is low (Savin and Epstein, 1970), δ18O of “altered” diatoms is likely to reflect the fractionation during formation of authigenic clays (e.g., glauconite, smectite and goethite) on top of and as a replacement of the original diatom frustule. This appears to be more likely to occur where the amount of clay to opal diatom-A is highest, perhaps as a result of higher reactivity of the diatoms with clays and porewater containing dissolved aluminum, magnesium and other ions. In this type of sediment, accumulation of diatoms will not result in chert formation. The δ18O of opal-CT differs from that of microquartz chert. In those cases, where the two phases co-exist, their δ18O reflects the respective phase of silica: opal-CT is consistently <25 ‰ and microquartz chert is about 20 ‰. For ODP 795, δ18O for opal-CT ranges from 20 to 28 ‰ and δ18O for microquartz chert ranges from 18 to 22 ‰. For ODP 799, δ18O for opalCT ranges from 24.9 to 25.3 ‰ and δ18O for microquartz chert ranges from 19 to 23 ‰. For example, at 577 mbsf in core ODP 799, the δ18O of opal-CT phase is 25.13 ‰ and the δ18O of microquartz chert phase is 20.42 ‰ (Fig. 5). This indicates that δ18O of the two different silica phases must be a consequence of dissolution-precipitation reactions of silica and not determined by precipitation of aluminum and iron oxide containing minerals. This coexistence of opal-CT and microquartz chert with different δ18O is observed in four other samples in both ODP 795 and 799 that exhibit the same δ18O difference between the two phases. Overall, our SEM/EDS and XPS results show that opal-CT and microquartz chert contain much lower aluminum and iron relative to “altered” diatom opal-A (Figs. 4B-D, 5F-H, 6B-D, 7C). These results agree with previously measured elemental compositions of opal-CT and microquartz chert (Iijima and Tada, 1981). δ18O of the most matured microquartz chert and its measured chemical composition confirms this differentiation between the formation of

14

the two phases (Fig. 6). Its δ18O is 21.64 ‰ with minor chemical impurities (Fig. 6B-D) further indicating that it must reflect the formation δ18O. In order to calculate the formation temperature of opal-CT and microquartz chert we use measured porewater δ18O for ODP 795 of -4.8 ‰ at the transition depth from opal-A to opalCT and of -4.95 ‰ at the transition depth from opal-CT to microquartz chert (Brumsack et al., 1992) and apply the Knauth and Epstein (1976) relation between temperature, δ18O of the water and the δ18O of the opal-CT and microquartz chert. We calculate formation temperatures of 36°C and 58°C, respectively. These are in good agreement with measured borehole temperatures, 43°C and 63°C, respectively (Fig. 8). We also use the observed measurements of porewater δ18O (Fig. 8A) (Brumsack et al., 1992) and geothermal temperature (Tamaki et al., 1990) (Fig. 8B) to calculate the expected equilibrium opal, opal-CT and microquartz chert, δ18O values (Fig. 8C), using the same equation from Knauth and Epstein (1976). We observe clear consistency between expected opal-CT and microquartz chert δ18O values and those that are measured (Fig. 8C) at the transition zones from opal-A to opal-CT and opal-CT to microquartz chert, respectively. Taking into account uncertainty in formation depths and porewater sampling resolution we consider these differences as reflecting the true variability of the sediment. Porewater δ18O for ODP 799 was not measured. However, Brumsack et al. (1992) measured porewater δ18O for a number of other sites within this leg (i.e., ODP 794, and 797) and observed that porewater δ18O at all ODP sites decreases with depth to values around -4 to -7 ‰ below 300 mbsf. Assuming a similar porewater profile for ODP 799, we calculate formation temperatures of 33°C for opal-CT and 62°C for microquartz chert, which are in the same range as calculated for ODP 795. We conclude that opal-A, opal-CT and microquartz chert are formed close to equilibrium with porewater δ18O and temperature. While diatom opal-A δ18O reflects ocean water characteristics (either δ18O of surface water and/or δ18O of surface sediment porewater and

15

temperature), opal-CT and microquartz chert δ18O reflect local geothermal gradient and downcore porewater δ18O profile. Considering that geothermal gradient and porewater δ18O vary on local to regional scale from 0 to -8‰ for Leg 127-128 (Brumsack et al., 1992), opal-CT and chert δ18O cannot be used to reconstruct past ocean temperature or seawater δ18O . We find that transformation of diatom opal-A to opal-CT occurs at about 40°C and the transformation of opal-CT to microquartz chert occurs at about 60°C. δ18O of opal-CT and microquartz chert is preserved through the sedimentary column. This suggests that opal-CT and microquartz chert form in isotopic equilibrium with their pore waters, and the respective isotopic compositions are subsequently resistant to isotopic alteration during further diagenesis on time scales of several million years. In environments where clay is the dominant lithology of the sediment, diagenetic path favors clay formation rather than opal-CT and microquartz chert. Given these conclusions, we compare our results to previously measured δ18O of opalCT and microquartz chert and consider implications for paleothermometry of the Miocene and Archean cherts. Murata et al. (1977) measured δ18O of diatomite, opal-CT, and microquartz chert in the Miocene Monterey Shale, deposited in marine conditions, but since, uplifted and exposed in Temblor Range, California. δ18O of diatom opal-A was 37.4 ‰, δ18O of opal-CT was 29.4±1.5 ‰, and of microquartz chert 23.8±0.3 ‰. The Monterey shale is exposed on land, hence also exposed to possible diagenetic alteration of the isotope signal by fresh water, with a much lower δ18O relative to seawater. Our sediments from a marine setting were never exposed to freshwater interaction and offer δ18O measurements that only record the original marine diagenesis without subsequent alteration. The fact that both in the Monterey Shale and in ODP cores 795 and 799, diatom δ18O values are very similar and there is a 5-6 ‰ difference between opal-CT and microquartz chert δ18O, suggests that contact with freshwater and aerial exposure did not alter the original Monterey δ18O of opal-CT and microquartz chert. The originally accumulated diatomite in the Monterey Shale also does not appear to have undergone

16

any diagenesis after aerial exposure as its δ18O of 37.4 ‰ is identical to the purest δ18O of diatoms measured in ODP 795 and 799 that are also of Miocene age. It points to the fact that on-land exposed diatomite (or spiculate and radiolarite) might preserve the original δ18O when buried, whether that of sea surface or bottom water or in topmost layer of the sediment. As onland diatomite, radiolarite, and spiculite deposits of older age are found elsewhere in the world (e.g., Cenozoic and older), this has paleoenvironmental implications for ocean paleothermometry. δ18O of diatom opal-A, opal-CT, and microquartz chert also have implications for Archean ocean paleothermometry. The maximum δ18O of Archean chert from Knauth and Lowe (2003) is 22.1 ‰, interpreted as indicating high temperatures of 55-85°C, considering the uncertainty in past seawater δ18O . If these δ18O measurements represent instead diagenetic δ18O, reflecting local temperature and porewater δ18O, with original δ18O being much higher, this suggests that Archean oceans may instead have been cooler. Without knowing porewater δ18O and temperature in which these cherts precipitated, it is impossible to draw conclusions about temperature of the Archean ocean. The possibility that it may not have been as hot as chert δ18O is originally interpreted should be considered. 4. CONCLUSIONS 1. δ18O of diatom opal-A can be interpreted paleoenvironmentally as long as the isolated silica is pristine, without any indication of authigenic mineral growth and/or replacement of the original silica skeletons by authigenic minerals. 2. Those diatoms that are deposited in high clay environments are more susceptible to clay alteration and will likely not become opal-CT and/or chert. 3. Opal-CT and microquartz chert form in sedimentary environments that are high in opal-A accumulation. We observe that diatom opal-A first dissolves and then re-precipitates as opal-CT. Formation of microquartz chert is a second dissolution / reprecipitation reaction.

17

4. The δ18O of Opal-CT and microquartz chert reflects the formation temperature and porewater δ18O and retains the isotope signature without post formation diagenetic alteration on time scales of millions of years. This is consistent with exposed opal-CT and microquartz chert Monterey Shales. 5. Formation temperatures of Opal-CT and microquartz chert occur at about 40°C and 60°C, corresponding to 300-600 mbsf in the specific region of the Sea of Japan.

Acknowledgements We thank the STEM Zuckerman Fellowship Program for fellow and research support, Irena Brailovsky for running laser fluorination line and assisting with isotope exchange of the studied material, and Kochi Core Center for allowing us to use sediments from ODP cores 795 and 799. This study was funded by a grant from the Israel Science Foundation to A.S and Y.K. and a partial support from the De Botton Center for Marine Research of the Weizmann Institute.

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Kolodny Y. and Epstein S., (1976) Stable isotope geochemistry of deep sea cherts. Geochimica et Cosmochimica Acta 40, 1195-1209. Labeyrie L.D. and Juillet A., (1982) Oxygen isotopic exchangeability of diatom valve silica; interpretation and consequences for paleoclimatic studies. Geochimica et Cosmochimica Acta 46, 967-975. Lancelot Y., 1973. Chert and silica diagenesis in sediments from the Central Pacific, U.S. Government Printing Office. Lawrence J.R., Gieskes J.M. and Broecker W.S., (1975) Oxygen isotope and cation composition of DSDP Pore Waters and the Alteration of Layer II Basalts. Earth and Planetary Science Letters 27, 1-10. Leng M.J. and Barker P.A., (2006) A review of the oxygen isotope composition of lacustrine diatom silica for paleoceanographic reconstruction. Earth Science Reviews 75, 5-27. Lynne B.Y., Campbell K.A., Moore J. and Browne P.R.L., (2005) Origin and evolution of the Steamboat Springs siliceous sinter deposit, Nevada, U.S.A. Sedimentary Geology 210. Marin J., Chaussidon M. and Robert F., (2010) Microscale oxygen isotope variations in 1.9 Ga Gunflint cherts: Assessments of diagenesis effects and implications for oceanic paleotemperature reconstructions. Geochimica et Cosmochimica Acta 74, 116-130. Marin-Carbonne J., Chaussidon M. and Robert F., (2012) Micrometer-scale chemical and isotopic criteria (O and Si) on the origin and history of Precambrian cherts: Implications for paleo-temperature reconstructions. Geochimica et Cosmochimica Acta 92, 129-147. Marin-Carbonne J., Robert F. and Chaussidon M., (2014) The silicon and oxygen isotope compositions of Precambrian cherts: A record of oceanic paleo-temperatures? . Precambrian Research 2014, 223-234. Matheney L.P. and Knauth L.P., (1993) New isotopic temperature estimates for early silica diagenesis in bedded cherts. Geology 21, 519-522. Meister P., Chapligin B., Picard A., Meyer H., Fischer C., Rattenwander D., Amthauer G., Vogt C. and Aiello I.W., (2014) Early diagenetic quartz formation at a deep iron oxidation front in the Eastern Pacific - A modern analague for banded iron / chert formations? Geochimica et Cosmochimica Acta 137, 188-207. Michalopoulos P. and Aller R.C., (2004) Early diagensis of biogenic silica in the Amazon delta: Alteration, authigenic clay formation, and storage. Geochimica et Cosmochimica Acta 68, 1061-1085. Michalopoulos P., Aller R.C. and Reeder R.J., (2010) Conversion of diatoms to clays during early diagenesis in tropical, continental shelf muds. Geology 28, 1095-1098. Mizutani S., (1977) Progressive ordering of cristobalitic silica in early stage of diagenesis. Contributions to Mineralogy and Petrology 61, 129-140. Murata K.J., Friedman I. and Gleason J.D., (1977) Oxygen isotope relations between diagenetic silica minerals in Monterey Shale, Temblor range, California. American Journal of Science 277, 259-272. Murata K.J. and Nakata J.K., (1974) Cristobalic stage in the diagenesis of diatomaceous shale. Science 184, 567-568. Murray R.W., Jones D.L. and Buchholtz ten Brink M.R., (1992) Diagenetic formation of bedded chert: Evidence from chemistry of the chert-shale couplet. Geology 20, 271274. Mustoe G.E., (2016) Density and loss on ignition as indicators of the fossilization of silicified wood. International Association of Wood Anatomics Journal 37, 98-111. Perry R.S., Lynne B.Y., Sephton M.A., Kolb V.M., Perry C.C. and Staley J.T., (2006) Baking black opal in the desert sun: The importance of silica in desert varnish. Geology 34, 537-540.

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Robert F. and Chaussidon M., (2006) A palaeotemperature curve for the Precambrian oceans based on silicon isotopes in cherts. Nature 443, 969-972. Savin S.M. and Epstein S., (1970) The oxygen and hydrogen isotope geochemistry of clay minerals. Geochimica et Cosmochimica Acta 34, 25-42. Schmidt M., Botz R., Stoffers P., Anders T. and Bohrmann G., (1997) Oxygen isotopes of marine diatoms: a comparative study of analytical techniques and new results on the isotope composition of recent marine diatoms. Geochimica et Cosmochimica Acta 61, 2275-2280. Sharp Z.D., Gibbons J.A., Maltsev O., Atudorei V., Pack A., Sengupta S., Shock E.L. and Knauth L.P., (2016) Calibration of the triple oxygen isotope fractionation in the SiO2H2O system and applications to natural samples. Geochimica et Cosmochimica Acta 186, 105-119. Shemesh A., Burckle L.H. and Hays J.D., (1995) Late Pleistocene oxygen isotope records of biogenic silica from the Atlantic sector of the Southern Ocean. Paleoceanography 10, 179-196. Shemesh A., Charles C.D. and Fairbanks R.G., (1992) Oxygen isotopes in biogenic silica: global changes in ocean temperature and isotopic composition. Science 256, 14341436. Tamaki K., Piscotto K. and Allan J., 1990. Site 795, Ocean Drilling Program, College Station, TX. Tartèse R., Chaussidon M., Gurenko A., Delarue F. and Robert F., (2017) Warm Archean oceans reconstructed from oxygen isotope composition of early-life remnants. Geochemical Perspectives Letters 3, 55-65. van Bennekom A.J., Jansen J.H., van der Gaast S.J., van Iperen J.M. and Pieters J., (1989) Aluminum-rich opal: an intermediate in the preservation of biogenic silica in the Ziare (Congo) deep-sea fan. Deep-Sea Research 36, 173-190. Williams L.A. and Crerar D.A., (1985) Silica diagenesis, II. General Mechanisms. Journal of Sedimentary Petrology 55, 312-321. Williams L.A. and Parks G.A., (1984) Silica diagenesis, I. Solubility Controls. Journal of Sedimentary Petrology 55, 301-311. Wilson M.J., (2014) The structure of opal-CT revisited. Journal of Non-Crystalline Solids 405, 68-75. Wise S., Buie B.F. and Weaver F.M., (1972) Chemically precipitated sedimentary cristobalite and the origin of chert. Eclogae Geologicae Helvetiae 65, 157-163.

Figure Captions Fig.1. Location of ODP 795 and 799 cores, Sea of Japan. The figure was made with Geomapp ap software.

Fig. 2. δ18O of silica as a function of depth within the ODP 795 and 799. Depth in mbsf is indicated on the left and age in millions of years (Myr) is indicated on the right. To the right

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of each set of δ18O measurements are lithological diagrams for each core from Kim et al. (2007) (A) δ18O of diatom opal-A, opal-CT, microquartz chert, and “altered” diatom opal-A with attached clays in ODP 795 on the left side of the figure. Respective burial temperatures are designated of 43ºC at a depth of 330 mbsf for transition of opal-A to opal-CT and 63ºC at a depth of 470 mbsf for transition of opal-CT to chert. Lithological composition of sediment is indicated on the right with the legend on the bottom right, originally composed from smear slides during the drilling expedition described originally in Tamaki et al. (1990). (B) Same as (A) but for ODP 799. Burial temperatures are 40ºC at a depth of 420 mbsf for transition of opal-A to opal-CT and 57ºC at a depth of 580 mbsf for transition of opal-CT to chert. The lithological composition, like for ODP 795, was composed from smear slides and described originally in Ingle et al. (1990).

Fig. 3. XRD spectra, δ18O, XPS, and SEM/EDS photograph and spectra for 799A-36X-2W (26-27 cm). (A)XRD of the sample, characterizing diatom opal-A. The main peak is centered at 22.2º in 2θ Cu K-alpha, is diffuse, and spreads 17 to 32º. Absence of any detrital quartz is recognized by absence of the 26.7º peak, attributed to quartz in the sample. (B) XPS analysis. Percentage corresponds to mass concentration of the sample for Si 2p, Al 2p, C 1s, O 1s, and Fe 2p spectral lines for three conditions of (1) no sputtering, (2) 1 minute sputtering, and (3) 3 minute sputtering. The three conditions constitute the x-axis. (C) SEM/EDS photograph with the qualitative documentation of the absence of elements outside of O and Si. (D) SEM/EDS spectra corresponding to the photograph in (C). Intensities of elements are indicated on the yaxis in units of cps/eV and identification of elements from their distinct energies is given in keV on the x-axis.

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Fig. 4. XRD spectra, δ18O, XPS, and SEM/EDS photograph and spectra for 795 H-2H-2W (2021 cm). (A) XRD of the sample, characterizing some peaks that correspond to low temperature α-quartz and also a number of others, corresponding to some combination of amorphous alumino-silicate authigenic minerals. These peaks are identified by letters I, S, K, and F. I refers to illite, S to smectite, K to kaolinite, and F to feldspar.

Observation of peaks

corresponding to α-cristobalite and α-tridymite are not observed. (B) XPS analysis. As in Fig. 2, the percentage corresponds to the mass concentration of the sample from Si 2p, Al 2p, C 1s, O 1s, and Fe 2p spectral lines.

(C) SEM/EDS photograph with low zoom and (D)

corresponding elemental spectrum. (E) SEM/EDS photograph with increased zoom indicating the remaining original structure of a diatom frustule circled in red from (C) and (F) corresponding elemental spectrum.

Fig. 5. Opal-CT transformation. Maturation is diagnosed with the XRD spectra, δ18O, XPS, and SEM/EDS photograph and spectra for 795 H-2H-2W (20-21 cm), both a sample that is identified to be more opal-CT like and that which has become more chert-like. The former is described in a through d and the latter in e through h. (A) XRD for opal-CT with clear peaks for low temperature α-cristobalite and α-tridymite at 22.2° and 36-37°. (B) XPS analysis. (C) SEM/EDS spectral elemental distribution for the surface of the sample and (D) corresponding spectrum. E) XRD for opal-CT with peaks corresponding to both cristobalite and tridymite at 22.2 and 36-37 but also, with a clearer emergence of the peak at 20.8 and 26.7, indicative of α-quartz chert mineralogy. Peak to the right of 26.7 α-quartz peak, corresponds likely to presence of feldspar. F) XPS. G) SEM/EDS spectral elemental distribution on the surface of the sample and H) corresponding spectrum.

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Fig. 6. XRD spectra, δ18O, XPS, and SEM/EDS photograph and spectra for 795B-18R-2W (1516 cm). (A) XRD of the sample, characterizing the complete maturation of biogenic opal-A to microquartz chert, with two distinct peaks at 20.8 and 26.7 indicative of α-quartz minerology. The 22.2 and 36-37 peaks belonging to opal-CT are very small. The small peak to the right of the α -quartz 26.7 peak as in fig. 3 and 4, we interpret to indicate to correspond to presence of feldspar. (B) XPS analysis. (C) SEM/EDS photograph with qualitative documentation of the presence of other elements. (D) SEM/EDS spectra corresponding to the photograph in (C).

Fig. 7. Characterization of silica maturation from diatom opal-A to microquartz chert with 18O, XRD, SEM/EDS. (A) Peak height ratio of two XRD peaks, 26.7 to 22.2 as a function of 18O. (B) FWHM of 22.2 peak in XRD as a function of 18O. (C) Elemental ratio from ratio of Al to Si peaks from SEM/EDS analysis. This is reported as mass concentration percentage. (D) 18O of opal-A, opal-CT, chert, and “altered” opal-A for ODP 795 and 799. Blue refers to opal-A, green to opal-CT, red to chert, and pink to “altered” opal-A. These data are provided in the Supplementary Materials 1 (SU 1). Closed symbols refer to data from ODP 795 and open symbols to data from ODP 799. Individual values are represented by triangles and average values for each phase are represented by squares. Standard deviation of the average is also reported and plotted with associated average values.

Fig. 8. (A) Measured porewater 18O from ODP 795 from Brumsack et al. (1992). (B) measured downhole temperature in middle (red contour) from Tamaki et al. (1990). (C) Calculated silica equilibrium 18O on the (black contour). The measured 18O values of the three different silica phases are plotted together with the silica equilibrium 18O. Silica equilibrium is calculated with the measured values of porewater 18O

and downhole

temperature using the relationship from Knauth and Epstein (1976). The two transitions zones 24

and corresponding downcore temperatures at which they occur are indicated by two black horizontal lines, one at 330 mbsf and corresponding to a temperature of 43C and one at 470 mbsf and corresponding to a temperature of 63C.

SU 1. Gives 18O and associated error, chertification, Al:Si, and FWHM measurements for ODP 795 and ODP 799; data for the two cores is separated into two tabs in the spreadsheet supplied. The samples are given the identification according to ODP classification and corresponding depth in meters below sea level (mbsf) and age in millions of years (Myr). Chertification refers to a ratio between peak heights of the α-quartz 10 1 (26.7) to αcristobalite 101 (22.2) in 2θ Cu K-alpha. Al to Si refers to a ratio of Al to Si from SEM/EDS work, calculated approximately and qualitatively. FWHM refers to width of the 101 peak at its half maximum height. Different phases of silica are separated by color: blue refers to diatom opal-A, green to opal-CT, red to microquartz chert, and pink to “altered” diatom opal-A. For ODP, 799, in the Age tab, LM refers to Late Miocene, MM refers to Middle Miocene, and EM refers to Early Miocene. The ages are general as the chronology developed from biostatigraphy suffered from a lack of well preserved microfossils (Ingle et al., 1990).

25

Sample name 795A-21X-3W (53-54 cm) 795A-35X-3W (80-81 cm) 795A-35X-4W (78-79 cm) 795A-35X-5W (95-96 cm) 795A-35X-6W (80-81 cm) 795A-34X-5W (25-26 cm) 795A-34X-6W (125-126 cm) 795A-35X-1W (55-56 cm) 795B-9R-1W (50-51 cm) 795B-10R-1W (12-13 cm) 795B-10R-2W (18-19 cm) 795B-10R-3W (118-119 cm) 795B-10R-6W (40-41 cm) 795B-11R-2W (78-79 cm) 795B-11R-3W (52-53 cm) 795B-11R-4W (15-16 cm) 795B-12R-1W (18-19 cm) 795B-12R-1W (30-31 cm) 795B-1R-1W (58-59 cm) 795B-11R-3W (52-53 cm) 795B-11R-4W (15-16 cm) 795B-12R-1W (30-31 cm) 795B-12R-3W (73-74 cm) 795B-14R-1W (68-69 cm) 795B-18R-2W (15-16 cm) 795A-2H-2W (20-21 cm)

Age (Myr) 3.18 5.22 5.24 5.26 5.28 5.11 5.15 5.17 7.86 8.13 8.18 8.25 8.36 8.49 8.52 8.56 8.72 8.76 5.93 8.52 8.56 8.76 8.72 9.22 10.06 0.23

ODP 795 Depth (mbsf) δ18O (‰)

Stdv. (‰)

Classification Diatom opal-A Diatom opal-A Diatom opal-A Diatom opal-A Diatom opal-A

185.14 321.41 322.89 324.55 325.91 314.12 316.69 318.16 442.91 452.13 453.69 456.19 459.91 463.99 465.23 466.36 471.21 473.21 365.79 465.23 466.36 473.21 475.14 491.39 530.76 10.95

37.17 36.19 34.71 34.15 34.60 24.38 27.74 27.15 25.02 19.60 20.48 23.86 24.81 25.65 25.41 24.77 26.51 24.76 19.95 21.43 21.93 18.19 19.82 17.64 21.64 21.72

0.83 0.71 0.40 0.80 0.14 0.34 0.44 0.56 0.08 0.90 0.84 0.55 0.82 0.45 0.34 0.55 1.83 0.45 0.34 0.30 0.24 0.48 0.66 0.71 0.71 0.52

795A-5H-2W (22-23 cm)

0.81

39.43

17.73

0.41

795A-6H-2W (20-21 cm)

0.98

48.93

21.42

0.22

795A-9H-2W (20-21 cm)

1.49

77.44

19.36

0.18

795A-11H-2W (17-18 cm)

1.78

96.40

19.57

0.81

795A-12H-2W (80-81 cm)

1.91

106.53

23.26

N/A

795A-14H-2W (56-57 cm)

2.16

125.26

21.49

0.32

795A-15H-2W (36-37 cm)

2.28

134.57

22.70

N/A

795A-26X-1W (110-111 cm)

3.93

231.21

32.15

0.45

795A-27X-1W (40-41 cm)

4.06

240.31

30.80

0.69

795A-29X-1W (131-132 cm)

4.35

260.82

33.08

0.75

795A-34X-1W (105-106 cm)

5.04

308.95

28.97

0.51

26

Opal-CT Opal-CT Opal-CT Opal-CT Opal-CT Opal-CT Opal-CT Opal-CT Opal-CT Opal-CT Opal-CT Opal-CT Opal-CT Microquartz chert Microquartz chert Microquartz chert Microquartz chert Microquartz chert Microquartz chert Microquartz chert “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A

Sample name 799A-30X-1W (28-29 cm) 799A-36X-2W (26-27 cm) 799A-38X-2W (56-57 cm) 799A-40X-2W (37-38 cm) 799B-9R-1W (47-48 cm) 799B-15R-1W (43-44 cm) 799B-16R-1W (23-24 cm) 799B-16R-1W (86-87 cm) 799B-15R-1W (43-44 cm) 799B-15R-2W (60-61 cm) 799B-20R-3W (59-60 cm) 799B-29R-1W (20-21 cm) 799B-46R-1W (20-21 cm) 799B-51R-1W (20-21 cm) 799B-53R-1W (24-25 cm) 799B-54R-1W (21-22 cm) 799A-19H-1W (105-106 cm)

Age (Myr) 3.90 4.32 4.36 4.40 LM2 MM3 MM6 MM7 MM3 MM4 MM18 MM23 MM25 MM26 MM27 MM28 2.35

ODP 799 Depth (mbsf) δ18O (‰)

Stdv. (‰)

Classification Diatom opal-A Diatom opal-A Diatom opal-A Diatom opal-A Opal-CT Opal-CT Opal-CT Opal-CT

223.87 318.67 338.22 357.43 519.48 577.44 586.84 587.47 577.44 579.11 628.80 712.00 875.51 923.57 942.85 952.42 165.81

35.86 37.57 37.02 35.86 25.30 25.13 24.89 24.96 20.83 21.40 20.35 18.92 22.56 20.57 20.97 20.25 32.97

0.70 0.56 0.41 0.37 N/A 0.76 0.51 0.21 0.34 0.63 0.82 0.54 0.57 0.81 1.57 0.77 0.84

799A-23X-2W (20-21 cm)

3.16

205.16

25.51

N/A

799A-25X-1W (108-109 cm)

3.38

223.87

24.32

N/A

799A-42X-1W (50-51 cm)

4.67

375.49

29.58

0.71

799A-43X-2W (50-51 cm)

4.95

386.59

24.27

1.14

799A-44X-1W (19-21 cm)

5.39

394.50

23.02

0.98

799A-45X-1W (43-44 cm)

5.55

404.34

21.11

0.43

799A-46X-2W (43-44 cm)

5.71

415.50

30.35

1.32

799A-46X-3W (37-38 cm)

5.72

416.91

26.03

0.79

799A-46X-4W (36-37 cm)

5.73

418.37

32.21

0.11

799A-46X-5W (27-28 cm)

5.74

419.75

30.16

0.45

799A-48X-1W (28-29 cm)

6.10

433.19

32.08

0.02

799A-48X-2W (23-24.5 cm)

6.12

434.64

21.11

0.18

799A-50X-1W (55-56 cm)

6.40

452.86

20.79

0.23

27

Microquartz chert Microquartz chert Microquartz chert Microquartz chert Microquartz chert Microquartz chert Microquartz chert Microquartz chert “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A “Altered” diatom opal-A