Paleoceanography of the northwestern Pacific during the Albian · Paleoceanography of the northwestern Pacific during the Albian Mitsuru Yamamuraa,, Hodaka Kawahataa,b,c, Katsumi

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Paleoceanography of the northwestern Pacific during the Albian

Mitsuru Yamamura a,⁎, Hodaka Kawahata a,b,c, Katsumi Matsumoto d,Reishi Takashima e, Hiroshi Nishi e

a Ocean Research Institute, The University of Tokyo, Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japanb Graduate School of Frontier Sciences and Ocean Research Institute, The University of Tokyo, Tokyo 164-8639, Japan

c National Institute of advanced Industrial Science and Technology (AIST), 1-1-1 Tsukuba-higashi, Ibaraki 305-8567, Japand Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, USA

e Department of Earth and Planetary Science, Graduate School of Science, Hokkaido University, N10 W8 Sapporo, 060-0810, Japan

Received 19 March 2006; received in revised form 2 May 2007; accepted 4 July 2007

Abstract

The deep ocean conditions and circulation during the Cretaceous have been characterized mainly by the sediments from theTethys Sea and the proto-Atlantic Ocean, because sedimentological data from the Pacific basin from that period have been quitelimited. Here we present new geochemical measurements from sediments that we collected from two sites in Hokkaido, Japan, thatwere presumably deposited in the northwest Pacific during the Albian (∼112 Ma). The low organic carbon and carbonate contents,combined with other measurements, from our study suggest that the mid-depth northwest Pacific during the time of deposition wascorrosive to sedimentary carbonate but oxic. We also use a simple box model of the ocean biogeochemistry to investigate theconditions of carbonate preservation and suggest that the northwest Pacific hydrography was quite distinct from the Tethys Sea andthe proto-Atlantic Ocean.© 2007 Elsevier B.V. All rights reserved.

Keywords: Albian; Northwestern Pacific; Geochemistry; Box modeling; Circulation

1. Introduction

The mid-Cretaceous represents one of the warmestclimate intervals during the entire Phanerozoic (Veizeret al., 2000). From the western North Atlantic Ocean(off Florida, ODP Site 1052E), Norris and Wilson(1998) report δ18O data from well-preserved planktonicforaminifera during the late Albian (∼100Ma) and earlyCenomanian (∼98 Ma) that were lower by −2.0‰ thanthat from benthic foraminifera. After taking into accountthe local salinity effect by the evaporation/precipitationand runoff, this indicates maximum sea surface temper-

atures (SSTs) of 31 °C, which is ∼6 °C higher than thatat an oceanic site at comparable latitude today. Also,intermediate-deep water temperatures are suggested tobe∼12 °C higher during 99–100Ma than that at presentaccording to a compilation of the foraminiferal δ18Odata from Ocean Drilling Program (ODP) and Deep SeaDrilling Program (DSDP) cores (Huber et al., 2002).Some believe that the mid-Cretaceous greenhousewarming may have been linked to the CO2 derivedfrom the enormous volcanic episode associated with theCretaceous superplume (e.g., Arthur et al., 1985; Vogt,1989; Larson, 1991). In this idea, the enhancedproduction of oceanic crust caused the sea level rise,which then reduced the albedo of the Earth's surface.The albedo change also may have intensified the

Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 477–491www.elsevier.com/locate/palaeo

⁎ Corresponding author. Fax: +81 3 5351 6438.E-mail address: [email protected] (M. Yamamura).

0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.palaeo.2007.07.002


positive feedback effect of the atmospheric and seasurface temperature (Jenkyns, 1999).

It is well known that the mid-Cretaceous in theTethyan basin and the proto-Atlantic Ocean is associ-ated with oceanic anoxic events (OAEs), which areperiods of basin-scale, elevated carbon burial in marinesediments (Schlanger and Jenkyns, 1976). Initially twoOAEs were identified for the Cretaceous: Aptian–Albian OAE 1 and OAE 2 at the Cenomanian/Turonianboundary (93.5 Ma). Subsequent investigations of OAE1 led to the recognition of three distinct sub-events ofocean-wide dysoxia/anoxia separated by oxic condi-tions: early Aptian OAE 1a (∼120 Ma), early AlbianOAE 1b (113–109 Ma), and 1d (99.5 Ma) (e.g., Arthur

et al., 1990; Jenkyns, 1991; Bralower et al., 1993;Erbacher and Thurow, 1998). In addition to these OAEs,the black layers of smaller regional extent arerecognized from the Tethys during the mid-Cretaceous(e.g., Bréhéret and Delamette, 1988; Erbacher andThurow, 1997). These imply that the entire TethysOcean had a potential to become dysoxic/anoxic duringthe times of these smaller depositions.

In the Pacific, available sedimentary data are mostlylimited to the equatorial region, and some of the Pacificdata also indicate burial of elevated organic carboncontent at times of OAE 1a and OAE 2 as in the Tethys(Meyers and Dickens, 1992; Dumitrescu and Brassell,2006). Outside the equatorial region, there is an isolated

Fig. 1. Map of the studied sections, central and northern Hokkaido, Japan. The sampling routes show the dashed quadrilateral.

478 M. Yamamura et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 477–491


study from the eastern Australia that suggests geochem-ical evidence in support of OAEs (Henderson, 2004). Inthe northwest Pacific, Hasegawa (1997) reportedfluctuations in carbon isotope values around theCenomanian–Turonian boundary that may be a globalsignal, although there was no concurrent burial ofincreased organic carbon content.

According to numerical modeling calculations in theglobal ocean circulation during the mid-Cretaceous,there are two modes for the location of the deep waterformation. Poulsen et al. (1999) suggest that deep waterforms only near the Antarctic while Bice and Norris(2002) suggest that deep water formation regions changefrom one hemisphere to the other hemisphere in responseto changes in atmospheric temperatures and hydrologicalcycles. In either case, the sea water has been subducted tothe deep sea floor in the Pacific Ocean. However, littlegeological evidence in the middle–high latitude on thePacific has been obtained during this interval.

Here we present various geochemical measurementsmade on terrestrial sediment samples collected fromcentral and northern Hokkaido that reflect marineconditions of the northwestern Pacific during the Albian.The aim of this study is to provide an implication for theglobal ocean circulation during the Albian. This regionwas the site of recent studies (Ando et al., 2002;Takashima et al., 2004) that produced stable carbonisotope stratigraphy and provided reliable, regionalgeological setting and age model. In an effort toreconstruct ocean chemistry and circulation in thenorth Pacific during the Albian, we compare our newdata to equivalent data from the Tethys and runsimulations using a simple three-box model of the ocean.

2. Geological setting

The studied sections are located in Hokkaido, anorthernmost island of Japan (Fig. 1). These sectionsbelong to the Sorachi-Yezo Belt in central Hokkaido.The range from the lower to upper Cretaceous of thisbelt is composed of the Yezo Group. The main parts ofthis group are stratigraphically continuous siliciclasticsuccessions of marine origin. The Yezo Group has athickness of over 8000 m, and much of the group wasformed some time between the early Cretaceous and thePaleocene. This group consists mainly of dark graycolored mudstone, terrigenous sandstone, and someconglomerates beds (Takashima et al., 2004). In thestudied area, the sedimentary strata strike north–south andhave an almost vertical dip that consists chiefly of tur-bidites and interbedded hemipelagic mudstone sequencesand is intercalated with a prominent olistostrome unit

(Takashima et al., 1997). The Yezo Group as a whole isinterpreted to have been deposited in a forearc basinsetting (Okada, 1983). It is suggested that the terrigenoussediments in this lower part of the Yezo Group weresupplied from Jurassic accretionary zone called OshimaBelt, which was located in northeast Eurasia (Kiminamiet al., 1992). The depositional environment of the YezoBasin was apparently deeper than the upper continentalslope on the basis of benthic foraminiferal assemblage(Motoyama et al., 1991; Takashima et al., 2004).

At our study sites, we follow the lithostratigraphy ofTakashima et al. (2004) at the Oyubari area andHashimoto et al. (1967) at Nakagawa area in centraland northern Hokkaido, respectively. In these areas, weexamined two sections at the Okusakainosawa River andChirashinaigawa River, respectively, where rocks arerelatively well-exposed and continuous (Figs. 1 and 2).As shown in Fig. 2, we refer to these two sections here asthe Okusakainosawa section and Chirashinaigawasection. Much of the Okusakainosawa section isrepresented by the Okusakainosawa Sandstone andMudstone Member, which is part of the ShuparogawaFormation in the lower part of the Yezo Group(Takashima et al., 2004). The Member consists ofmudstone-dominant alternating beds between olistos-trome occasionally including limestone olistoliths withlarger foraminifer Orbitolina (Kirigishiyama Olistos-trome Member; e.g., Sano, 1995) and felsic volcaniclas-tic sandstones, tuffs and conglomerates (MaruyamaFormation). Takashima et al. (2004) suggests that theAptian–Albian boundary is within the KirigishiyamaOlistostromeMember, so the horizon that corresponds tothe age of OAE 1b at the Tethys is missing in theOkusakainosawa section. The age of the Okusakaino-sawa Sandstone and Mudstone Member is interpreted asthe early Albian based on a carbon isotope stratigraphyfrom terrestrial organic carbon and on microfossilanalysis (Ando et al., 2002; Nishi et al., 2003; Takashimaet al., 2004). In consideration for 500 m thick of theMember, the average of the sedimentation rate in thisinterval was estimated at ∼3.5 cm per kyr.

The Chirashinaigawa section is divided into threeformations in ascending order: Kamiji Formation,Moihoro Formation, and Shirataki Formation, asdescribed by Hashimoto et al. (1967) (Fig. 2). KamijiFormation is composed of dark gray mudstone andturbiditic sandstone and lithologically divided to eightsub-units (Kj1–Kj8). Most parts of Moihoro Formationare made of tuffaceous sandstone with turbiditic facies.Finally, Shirataki Formation is composed mainly of thedark gray mudstone with rhythmic intercalated beds ofturbiditic sandstone, mudstone and some tuffaceous

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Fig. 2. Simplified columnar sections at two field sites. Age assignment is based on microfossil (Takashima et al., 2001) and carbon isotopestratigraphies (Ando et al., 2003) at Okusakainosawa Section and ammonites and microfossils dating by Matsumoto (1984) and Iba et al. (2005).



sandstone. The age of the Chirashinaigawa section is notwell-constrained, because there is little occurrence offossils to determine the biostratigraphy. Recently, Ibaet al. (2005) reported the existence of the carbonatepebbles containing Orbitolina from the lowermost partof the Kamiji Formation in Okahonai River at ∼3 kmnorth from Chirashinaigawa section and indicated thatthis layer is comparable to the Kirigishiyama Olistos-trome Member at the Oyubari area. This suggests thatthe lowermost part of the Chirashinaigawa sectioncorresponds to the early Albian. This age is consistentwith the occurrence of late Albian macrofossils in theupper part of Shirataki Formation (Hashimoto et al.,1967). In addition, radiolarian biostratigraphy examinedat this area is consistent with these results (e.g., Mitsugiand Hirano, 1998).

3. Materials and methods

3.1. Materials

Sedimentary samples for geochemical analyses werecollected in the Okusakainosawa and Chirashinaigawasections along the respective rivers (Fig. 1). All sampleswere collected from the horizon where hemipelagicmudstones were exposed at two sections. Turbidite andother deposits were not taken. For elemental analysis, 85samples were collected at intervals of 0.3–80 m from theoutcrop along the Okusakainosawa River and 212samples at intervals of 0.3–350 m along the Chira-shinaigawa River. Of these, 42 samples were selectedfrom the Okusakainosawa River and 67 samples fromthe Chirashinaigawa River for analysis for the totalorganic carbon content.

3.2. Analytical procedure

All sediment samples were crushed to fine powderand split into several subsamples for elemental analyses.Inorganic element contents were analyzed using Induc-tively Coupled Plasma Atomic Emission Spectrometer(ICP-AES). One hundred milligrams of each dry bulksample was dissolved by an ultrapure mixture of HF,HCl, HNO3, and H2SO4. The solutions were thendiluted with 5 ml of HNO3 and distilled MilliQ-water toadjust the volume to 100 ml. The solution samples thusprepared were analyzed by a Rigaku JY-38 ICP analyzerat the Geological Survey of Japan. Reference rockstandards were provided by the Geological Survey ofJapan (JB-2, JA-2, JG-1a, and JR-1) as well as standardsolutions prepared from pure elemental standard solu-tions were used for calibration. Analytical error of each

elemental analysis by the ICP-AES was estimated to beless than 3%.

Total carbon content of the sediment samples wasdetermined with a Yanako MT-5 CHN analyzer at theGeological Survey of Japan. Analytical precision isapproximately ±2%. Carbonate content was determinedby measuring Ca concentration in a solution afterdecalcification of the sediment samples with 1% aceticacid by 1 h. The calcium content derived from carbonatewas analyzed by ICP-AES. Analytical precision is about±3%. Organic carbon content was defined as thedifference between total carbon content and carbonatecarbon content.

3.3. Model description

In our numerical simulations, we used a simple oceancarbon cycle model based on the three-box model ofToggweiler and Sarmiento (1985) (Fig. 3). The threeboxes represent low-latitude surface ocean, high-latitude surface ocean, and deep ocean, which areindicated with subscripts l, h, and d respectively.Approximately 15% of the world ocean surface area isrepresented by the high-latitude box. Variable Trepresents the large scale meridional overturning ofthe world ocean. Polar vertical mixing fhd is meant torepresent deep water ventilation and formation aroundAntarctic continent today.

The model has five state variables: phosphate,oxygen, potential temperature, alkalinity, and ∑CO2

(Table 1). Salinity is fixed everywhere to 35 practical

Fig. 3. Schematic diagram of three-box model. l, h, and d boxesrepresent low-latitude nutrient-depleted surface sea, high-latituderegion formed deep water and deep ocean, respectively. CO2 and O2

gas are exchanged between atmosphere and sea surface. Large arrows(T) indicate thermohaline circulation. Small arrows (fhd) stand forventilation at high-latitude region. Wiggle arrows indicate an exportproduct controlled by phosphate in this model.



salinity unit (psu), to which phosphate, alkalinity, and∑CO2 are normalized. Temperature, 35-psu salinity,alkalinity, and ∑CO2 are used to compute the solubilityand the disassociation constants of the carbon systemand pCO2 under the condition of chemical equilibrium(Dickson and Goyet, 1994). Surface temperatures areprescribed to their initial values. Gas exchanges of CO2

and oxygen with the atmosphere are driven by therespective partial pressure gradients across the air–seainterface and a “piston velocity” of 3 m day−1 (Broeckerand Peng, 1982). We define the carbonate saturationdepth by the intersection of [CO3

2−] in the deep box and[CO3

2−]sat (the carbonate ion concentration that is atsaturation with respect to mineral calcite) calculatedfrom the apparent solubility product Ksp for calcite.

Phosphate serves as the main nutrient that accountsfor organic carbon production. The phosphate concen-tration in the low-latitude surface box drives its newproduction by restoring the phosphate concentration toits initial value. The production rate of organic carbonfrom the surface ocean is expressed as the flux ofphosphate, which is assumed to occur with a stoichi-ometry of P:Corg:−O2=1:117:170 (Anderson and Sar-miento, 1994). In this model, the new production rate atthe low-latitude surface box, which is described as Pl,depends on the upwelling supply flux of phosphate dueto the thermohaline circulation. In the high-latitude box,the new production rate is fixed to a prescribed value,since the productivity seems to be affected to factorsother than nutrients (Sarmiento et al., 1988). With theproduction rate, we adjust a global average export ratioof CaCO3:Corg to 0.1 based on Sarmiento et al. (2002).

4. Results and discussions

4.1. Characteristics of the organic matter in the studiedarea

The measured total nitrogen content ranged between0.04 and 0.08 in unit of weight percent (wt.%) (Fig. 4).This includes inorganic N and/or decomposed organic Nabsorbed into the clay mineral (Müller, 1977). Total

organic carbon content in the same sections rangedbetween 0.3 and 1.2 wt.% (Fig. 4).

Since organic matters produced at sea surfacegenerally show lowCorg/N atomic ratio (∼6–7), whereasorganic matters supplied from terrestrial usually showratios of 20 and higher value (Hedges et al., 1986), theatomic ratio is often used to infer the origin of organicmatter as in a case of two endmember mixing. In theOkusakainosawa section, the bulk Corg/N ratio isbetween 6.7 and 19.4 (Fig. 4A). The ratio is generallyhigh when Corg content is high, which indicates thatwhen there is a large supply of organic matter, it ismainly derived from land as opposed to the oceans. Thisis corroborated in Fig. 5A, which shows that Corg and Nhave a strong correlation (r=0.87) with a slope of 23.3 interms of atomic ratio (Fig. 5A). The high ratio is moreconsistent with a terrestrial source of organic carbon thana marine source. This regression line does not passthrough the origin in Fig. 5A but intersects the horizontalaxis (Corg=0 wt.%) when the nitrogen content is0.025 wt.%, so that the ratio of 23.3 revealed in Fig.5A is slightly higher than the bulk ratio as shown Fig.4A. This amount likely represents inorganic nitrogen,decomposed organic nitrogen absorbed into clay, orsome combination of the two (Müller, 1977). Additionalsupport for the dominant input of terrestrial sediments tothe Okusakainosawa section is provided by measure-ments of aluminum (Al) content, which is stronglycorrelated with organic carbon content (r=0.77; Fig.5B). As shown by Duce et al. (1983), the presence ofaluminum in marine sediments suggests an input ofterrestrial materials. In addition, some occurrences ofturbidite layers in the section mean that Al was suppliedmainly by river input. Therefore, it is most natural toinfer that most organic carbon was transported withaluminosilicate minerals by rivers to the Okusakaino-sawa section. These observations are also consistent witha previous microscopic examination of kerogen (Hase-gawa, 1997; Ando et al., 2003) and with recent studiesthat suggest that the Okusakainosawa section reflects ahemipelagic depositional environment (Motoyama et al.,1991; Takashima et al., 2004).

Table 1Steady states for model

Boxes °C PO4− μmol kg−1 O2 μmol kg−1 ∑CO2 μmol kg−1 Alkalinity μmol eq kg−1

OceanLow 21.5 0 219 1989 2340High 2.0 1.5 332 2185 2376Deep 2.0 2.2 210 2279 2394

Atmosphere 280Inventory 2.80×1015 3.06×1018 3.17×1018



Fig. 4. Corg/N atomic ratio and content of Corg, nitrogen, carbonate, Al and Mn at the Okusakainosawa Section (A) and Chirashinaigawa Section (B).The Shading indicates the range of manganese content described by Kawamura et al. (2000). These symbols indicate Corg/N atomic ratio (filleddiamond), organic carbon (filled square), nitrogen (cross), carbonate (open square), aluminum (filled circle) and manganese (open triangle).



In the Chirashinaigawa section located further tothe north, the bulk Corg/N atomic ratio is generallyslightly lower than those observed in the Okusakai-nosawa section (Fig. 4B). Organic matter contentincreases as a function of total nitrogen but with aslope converted to Corg/N atomic ratio of about 13(Fig. 5C). Together, they suggest that organic matterin the Chirashinaigawa reflects a greater contributionof marine source than terrestrial source when com-pared to the more southerly located section. This isconsistent with the observation that organic carboncontents have a weaker correlation with Al contents atthe Chirashinaigawa (Fig. 5D). Nevertheless, a Corg/Nratio of 13 (Fig. 5C), which is still significantlyhigher than the typical marine endmember of 6–7,suggests that terrestrial organic matter had been animportant contributor to the Chirashinaigawa sectionas well.

4.2. Redox condition

Manganese (Mn) has three oxidation states: Mn(II),Mn(III), and Mn(IV). Generally, Mn concentrations insediments are ruled by lithogenics in anoxic basinsbecauseMn oxyhydroxides (Mn(III), Mn(IV)) depositedon the sea floor are reduced to soluble Mn(II) whichdiffuses out into the overlying sea waters. Redoxcondition is one of the more telling environmentalparameters to understand the biogeochemical conditionsof the Cretaceous ocean, because as mentioned before,anoxic conditions are often inferred in the Tethys and theproto-Atlantic Ocean during the Albian. However, as yetfew sets are available from the Pacific for this period.

Mn concentrations varied from 266 ppm to 820 ppmwith a mean value of 466 ppm in the Okusakainosawasection (Fig. 4A), and from 254 ppm to 2040 ppm with amean value of 667 ppm in the Chirashinaigawa section

Fig. 5. Correlational diagram of organic carbon content versus nitrogen (A, D) and aluminum versus organic carbon content (B, E) and manganese(C, F). Circles indicate the data at the Okusakainosawa section, central Hokkaido and triangles indicate the data at the Chirashinaigawa section,northern Hokkaido.



(Fig. 4B). The bulk Mn concentration of sandstonesfrom the Oshima Belt, which is considered a plausiblesource of silty or clay minerals to both sections(Kiminami et al., 1992), is roughly between 100 and300 ppm (Kawamura et al., 2000) and somewhat lowerthan in our study sections. There is a lack of strongcorrelation between Mn and Al contents in both sections(Figs. 6A and B), which may simply mean that Mn andAl were not contained in the same terrigenous minerals.This may mean that Mn oxyhydroxides were incorpo-rated in the sediment through the supply of manganesefrom the water column via the scavenging effect. Thefact that Mn concentrations in the two sections are onlysomewhat higher than the potential source rockssuggests that the dissolved oxygen in the bottom waterwas not completely consumed during the Albian, whenboth the Okusakainosawa and Chirashinaigawa sectionswere formed on the seafloor. This is also inferred fromthe low organic carbon content in these sections.

Mn concentration in the study area shows weaknegative correlation with organic matter (Figs. 6C and D).In addition, maximum peak of Mn often occurred justabove maximum peak of organic matter (Fig. 4). Thissuggests that Mnmigrated in the sediments in response tochanging redox conditions and repeated dissolution/precipitation. However, the presence of organic carbonin low abundance in both the Okusakainosawa andChirashinaigawa sections indicates that these sedimentsaccumulated largely under oxic conditions and Mn mightmigrate through the sediments under reduced conditionafter burial.

4.3. Carbonate preservation

As shown in Fig. 4, both sections have very littlecarbonate, although the carbonate content increasessporadically to as high as ∼9 wt.%. Since carbonatepreservation in the sediments is controlled by dilution

Fig. 6. Correlational diagram of manganese versus aluminum (A, C) and organic carbon content (B, D). Circles indicate the data at theOkusakainosawa section, central Hokkaido and triangles indicate the data at the Chirashinaigawa section, northern Hokkaido.



effect due to the inputs of terrigenous matters, carbonateproduction at the sea surface, and by its subsequentdissolution in the water column and sediments, the lowcarbonate content found in these two sections may beattributed to these reasons.

In order to consider the dilution effect by terrige-nous matters in the studied sections, we compare thesedimentation rate and carbonate content in the sectionwith DSDP Site 1049 off Florida (Erbacher et al.,2001). At Site 1049 the sedimentation rate shows∼1.0 cm kyr−1 and carbonate content is between 15 and80 wt.% during the Albian. On the other hand, in theOkusakainosawa section the sedimentation rate afterexcluding the thickness of turbidite sandstone layersshows ∼3.5 cm kyr− 1 and carbonate content isbetween 0 and 4.6 wt.%. On the assumption that thedifference of sedimentation rate at these is attributed tothe amount of the terrigenous matters, the carbonatecontent in the Site 1049 divided by 3.5 is between 4.3and 22.9 wt.%. This value is apparently higher than

that at studied section. Precisely, it is hardly difficult tomake the carbonate content decrease to 0 wt.% by onlydilution effect. Thus, we acquire considering the otherfactor for the low carbonate content. Then, stableoxygen isotope records from multiple species of well-preserved foraminifera suggest that the thermal structureof surface waters in the western tropical Atlantic Oceanunderwent pronounced variability, with maximum seasurface temperatures 3–5 °C warmer than today (Wilsonand Norris, 2001). Based upon the modern sediment trapexperiments, carbonate is generally the largest compo-nent of settling particles (Kawahata, 2002), except insubarctic gyre, where sea surface temperature is quitelow. Therefore, it is unlikely that marine carbonate wasnot produced in the surface ocean in the Albian. Actually,carbonate deposition has been recognized at 40°N inTethys during Cretaceous, and it has been suggested thatthe sedimentary basin of the studied area had been locatedat b40°N in the western Pacific (Hoshi and Takashima,1999). Therefore, it is natural that little occurrence of

Fig. 7. Three-box model sensitivity of saturation depth and carbonate ion concentration at deep box. (A) Ventilation fluxes fhd (Sv) versus T (Sv).(B) Atmospheric pCO2 versus temperature at high box. (C) Temperature at high box versus alkalinity. Solid line indicates carbonate ionconcentration (μmol/kg) and broken line indicates saturation depth (m). In Fig. 6A, the initial value of ventilation fluxes is indicated by the starsymbol. The light-gray shading indicates the saturation depth below 0 m. The dark shading indicates the range of the saturation depth similar to theinterglacial value.



carbonate in the sections is due to dissolution through thewater column and/or on the seafloor. Because the mid-Cretaceous apparently had much higher carbon dioxideconcentration in the atmosphere than today (e.g., Bernerand Kothavala, 2001), seawater carbonate chemistrywould predict that the oceans then were more acidic thanoceans today. This would in turn suggest that calciumcarbonate was much easier to dissolve in the watercolumn. In order to investigate this possibility, we use asimple three-box model of ocean carbon cycle.

In initial values which represent the modern condi-tions, the model has a carbonate ion concentration ofabout 94 mol kg−1 and 2 °C in the deep box. Thecalcium carbon saturation horizon under these modelconditions is located at the depth of about 3700 m. Wefocus on the saturation horizon, because it is an index ofhow corrosive the deep ocean is to calcium carbonate.

We first show the sensitivity of the saturation horizonto two physical parameters T, fhd (Fig. 7A). Here T andfhd are changed in the range of 0–50 and 0–100 Sv,respectively. As deep ocean ventilation is enhanced (i.e.,larger fhd), the saturation horizon is deepened. All themodel experiments show that the saturation depth isdeeper than 3500 m, except when fhd is lower than

10 Sv. Next, we consider the higher concentrations ofthe atmospheric pCO2 and warmer polar (high-latitudesurface box) conditions during the mid-Cretaceous(Fig. 7B). The saturation horizon is much more sensitiveto these parameters than to the two previous physicalparameters. We see that the saturation horizon shoalswith increasing atmospheric pCO2 and decreasingtemperature. For modern polar surface temperature of2 °C, the saturation horizon reaches the surface oceanwith just twice the modern pCO2. The same is observedat 20 °C and five times the modern pCO2. When thesaturation horizon reaches the surface, the entire oceanis undersaturated in carbonate ion content with respectto calcium carbonate. The mid-Cretaceous is believed tobe 6–12 °C warmer than today (Barron et al., 1995) andhad 2–10 times more atmospheric CO2 (Berner andKothavala, 2001). During the early Albian, a part of theKerguelen Plateau was erupted above sea level (Schlichet al., 1989), which might result in high atmosphericpCO2. Thus, our model results would indicate thatcarbonate dissolution in the northwestern Pacific duringthe Albian was severe. However, this is contrary to thegeological observations that calcium carbonate wasapparently present at paleodepths of ∼1000–2000 m in

Fig. 7 (continued ).



DSDP/ODP cores (e.g., Dean, 1981; Duval et al.,1984; Norris et al., 1998) and the sediment was mainlycomposed of the carbonate except for OAE 1b intervalin the equatorial Pacific (Robinson and Williams,2004).

This apparent contradiction may be resolved if therewere more alkalinity in the oceans during the Albianthan today. Global warming with high atmosphericpCO2 like the Albian would have induced intensifiedhydrological cycles on the Earth's surface (e.g., Jenkyns,1999). Subsequently, this would have resulted in theintensified weathering and increased supply of nutrientsand alkalinity. With a greater amount of alkalinity andthus buffering capacity, the saturation horizon would notshoal in response to higher temperatures and atmosphericpCO2 as shown in Fig. 7B. This would allow somecarbonate to be deposited on the sea floor as indicated byour measurements (Fig. 4). A higher alkalinity inventoryin the Albian oceans is in fact expected by anenhancement of chemical weathering of silicate rocks,driven by a higher carbon dioxide content in theatmosphere (e.g., Broecker and Sanyal, 1998; Cohenet al., 2004).

In the late Albian, the deep sea temperature isestimated at ∼15 °C in the equatorial Pacific (Huber

et al., 2002) and atmospheric CO2 concentrations aresuggested to vary between ∼2 and 5 times more thanthat at present atmospheric pCO2 (Bice et al., 2006). Toinvestigate the effect of a higher alkalinity content in theocean, we conduct a set of model experiments with thesame range of high-latitude surface box temperature andfour times the present atmospheric pCO2 (Fig. 7C).Within the given parameter space, our results show thatthe ocean alkalinity inventory has to be at least 1.3 timesthe present inventory in order to maintain the presentdepth of saturation horizon, as indicated by the darkshading in Fig. 7C. The required polar surfacetemperature in this case is 20 °C, which is likely toohigh. For a smaller temperature increase, the requiredalkalinity inventory to maintain the present level ofsaturation depth is greater than 1.2 times.

Larger oceanic alkalinity content can also be arguedfrom the perspective of the production of calcifyingorganisms at the surface. Recently Zondervan et al.(2001) pointed out that an acidification of the oceanslows or prevents growth of calcifying primary produ-cers. So the fact that we see some carbonate preservationduring the Albian (Fig. 4) suggests that the ocean surfacewas sufficiently basic, despite higher atmospheric pCO2,to allow growth of calcifying producers.

Fig. 7 (continued ).



4.4. Implication for ocean circulation during the Albian

While our new carbonate data from Hokkaidoindicate that the northwest Pacific during the Albianhad poor carbonate preservation, roughly coeval sedi-ments collected from the equatorial Pacific (DSDPHole 463, paleodepth 500–1000 m, paleolatitude ∼0°)show high abundance in carbonate (Dean, 1981). Ifwe assume that the extent of carbonate preservation inthe two regions largely reflects preservation and thatthe paleodepths at the two regions are similar, then thegradient in carbonate preservation may suggest thatthe northwest Pacific was located downstream relativeto the equatorial region in terms of ocean circulation.As in modern oceans, interior water is expected tobecome increasingly acidic, as more sinking particlesdissolve.

Available geochemical evidence, including our Mnmeasurements, from the two Albian Pacific regions doesnot indicate anoxia. This may suggest that local organiccarbon production and rain were not sufficient to depleteoxygen at the seafloor, even though sediments from bothregions have accumulated in paleodepths that coincidewith today's oxygen minimum zone.

One of the most interesting features of the anoxicsediments in the Tethys is that large quantities of organiccarbon were often deposited in association withcarbonates (e.g., nannoconid, foraminifera). In fact,the well-preserved planktonic foraminifera have beendiscovered from the black shale collected from ODPSite 1049 in Tethys (Norris and Wilson, 1998). The factthat carbonate is not significantly dissolved even insuboxic or anoxic waters may be because the alkalinitycontent in the Albian ocean was indeed much larger, aswe suggested earlier. Possible explanations for anoxiccondition are: (1) abundant local rain of organic matterconsumed dissolved oxygen in the water column,(2) ventilated waters had equilibrated at the surface attemperatures 3–5 °C higher than those of modern ocean(Wilson and Norris, 2001) and so saturation oxygencontent was much lower.

In terms of carbonate preservation and dissolvedoxygen content, the contrast between the northwestPacific and Tethys is interesting. In the northwest Pacific,dissolved oxygen was likely not consumed but carbonatewas often mostly dissolved. In the Tethys, dissolvedoxygen was depleted but carbonate was well preserved.These differences, if real, may be explained by thepossibility that the Pacific and Tethys had distinctsources and thus chemical composition of intermediateand deep waters during the Albian. To examine thishypothesis, it is necessary to accumulate data of the

northwest Pacific area by Integrated Drilling OceanProgram (IODP).

5. Conclusions

We examined two sedimentary sections from Hok-kaido, Japan that were formed in the northwest Pacificduring the Albian. On the basis of Corg/N atomic ratios,terrestrial organic carbon was apparently a majorcontributor of organic matter in the silty rocks of theOkusakainosawa and Chirashinaigawa sections. Mea-surements of Mn indicate the presence of oxygen, butthere was very little carbonate. Poor preservation ofcarbonate may be an expected consequence of highatmospheric CO2 content during the mid-Cretaceous, ashigh atmospheric CO2 content would acidify the ocean.However, we cannot rule out the possibility that therewas reduced carbonate production at the surface. In anycase, our model results indicate that under highatmospheric CO2 content during the mid-Cretaceous,an increase in the oceanic inventory of alkalinity is likelynecessary to preserve any carbonate on the seafloor.Otherwise, the entire ocean may be undersaturated withrespect to calcium carbonate. Last, we note a potentiallyimportant difference between the Pacific and Tethys.Dissolved oxygen was largely consumed and carbonatewas well-preserved in the Tethys but not in the Pacific. Ifreal and significant, this difference may argue that thetwo basins had distinct sources of intermediate and deepwaters and that the global overturning circulation duringthe Albian was quite different from that of today.

Acknowledgements

The authors wish to thank K. Minoura of (TohokuUniversity) and Lallan P. Gupta (CDEX) for helpfulsuggestions, M. Nohara (AIST) and K. Minoshima(AIST) for their technical assistance in geochemicalanalysis, and F. Surlyk and anonymous reviewers for theirreviews. Special thanks are also due to K. Kobayashi(Mikasa City), his family, Y. Hikida, T. Matsuda(Nakagawa Museum of Natural History), and colleaguesfor their kind supports during the fieldwork. Additionalsupport was provided by the Nippon Foundation HadalEnvironmental Science/Education Program (HADEEP).This study was supported by the following researchprograms: “GCMAPS Program (Global carbon cycle andrelated mapping based on satellite imagery)”, funded bythe Ministry of Education, Culture, Sports, Science andTechnology of Japan, and the grant-in-ad for scientificresearch of 17253006 and 16340161 funded by the Japansociety for the promotion of science.



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Paleoceanography of the northwestern Pacific during the Albian · Paleoceanography of the northwestern Pacific during the Albian Mitsuru Yamamuraa,, Hodaka Kawahataa,b,c, Katsumi