(Table 1) Age of Cretaceous seamounts in the Western Pacific

Dynamics of the Pacific Plate is recorded in the systematic variation of location and the 40Ar-39Ar age of seamounts in the Western Pacific from 120 to 65 Ma ago. The seamounts are grouped into three linear zones as long as 5000 km. The seamounts become younger in the southeastern direction along the strike of these zones. Correlation between age and location of seamounts allows to divide the history of their formation into three stages. Rate of seamount growth was relatively low (2-4 cm/yr) during the first and the third stages within intervals of 120-90 and 85-65 Ma, whereas during the second stage (90-85 Ma), the seamounts were growing very fast (80-100 cm/yr). In the midst of this stage, at ~87 Ma ago, magmatic activity increased abruptly. Dynamics of seamount building is in good agreement with (1) pulses in development of the Ontong Java, Manihiki, and Caribbean-Colombian oceanic plateaus; (2) age of spreading acceleration in the mid-Cretaceous; and (3) a short period when the Izanagi Plate ceased to exist and the Kula Plate was formed. Variation in seamounts' age and location are in consistence with the hypothesis of diffuse extension of the Pacific Plate in course of its motion with formation of impaired zones of decompression melting. Direction of extension (325°-340° NW) calculated from the strike of seamount zones is consistent with the path of the Pacific Plate (330° NW) in the Late Cretaceous. Immense perioceanic volcanic belts were formed at that time along the margin of the Asian continent. The Okhotsk-Chukchi Peninsula Belt extends at a right angle to the compression vector. Three stages of this belt's evolution are synchronous with the stages of seamount formation in the Pacific Plate. Delay in origination of the East Sikhote-Alin Volcanic Belt and its different orientation were caused by counterclockwise rotation of the vector of convergence of oceanic and continental plates in the mid-Cretaceous. At the same time, i.e. 95-85 Ma ago, volcanic activity embraced the entire continental margin and tin granites were emplaced everywhere in the Eastern Asia. This short episode (90+/-5 Ma) corresponds to the mid-Cretaceous maximum of compression of the continental margin, and its age fits well a culmination in extension of the Pacific Plate.

(Table 1) n-alkane distribution, and stable carbon and deuterium isotope ratios of DSDP Hole 10-94

Hydrology, source region, and timing of precipitation are important controls on the climate of the Great Plains of North America and the composition of terrestrial ecosystems. Moisture delivered to the Great Plains varies seasonally and predominately derives from the Gulf of Mexico/Atlantic Ocean with minor contributions from the Pacific Ocean and Arctic region. For this work, we evaluate long-term relationships for the past ~ 35 million years between North American hydrology, climate, and floral change, using isotopic records and average carbon chain lengths of higher plant n-alkanes from Gulf of Mexico sediments (DSDP Site 94). We find that carbon isotope values (d13C) of n-alkanes, corrected for variations in the d13C value of atmospheric CO2, provide minor evidence for contributions of C4 plants prior to the Middle Miocene. A sharp spike in C4 input is identified during the Middle Miocene Climatic Optimum, and the influence of C4 plants steadily increased during the Late Miocene into the Pleistocene - consistent with other North American records. Chain-length distributions of n-alkanes, indicative of the composition of higher plant communities, remained remarkably constant from 33 to 4 Ma. However, a trend toward longer chain lengths occurred during the past 4 million years, concurrent with an increase in d13C values, indicating increased C4 plant influence and potentially aridity. The hydrogen isotope values (dD) of n-alkanes are relatively invariant between 33 and 9 Ma, and then become substantially more negative (75 per mil) from 9 to 2 Ma. Changes in the plant community and temperature of precipitation can solely account for the observed variations in dD from 33 to 5 Ma, but cannot account for Plio-Pleistocene dD variations and imply substantial changes in the source region of precipitation and seasonality of moisture delivery. We posit that hydrological changes were linked to tectonic and oceanographic processes including the shoaling and closure of the Panamanian Seaway, amplification of North Atlantic Deep Water Production and an associated increase of meridional winds. The southerly movement of the Intertropical Convergence Zone near 4 Ma allowed for the development of a near-modern pressure/storm track system, driving increased aridity and changes in seasonality within the North American interior.

Organic matter composition of Cretaceous sediments of the central Pacific Ocean

Complete records of organic-carbon-rich Cretaceous strata were continuouslycored on the flanks of the Mid-Pacific Mountains and southern Hess Rise in the central North Pacific Ocean during DSDP Leg 62. Organic-carbon-rich laminated silicified limestones were deposited in the western Mid-Pacific Mountains during the early Aptian, a time when that region was south of the equator and considerably shallower than at present. Organic-carbon-rich, laminated limestone on southern Hess Rise overlies volcanic basement and includes 136 m of stratigraphic section of late Albian to early Cenomanian age. This limestone unit was deposited rapidly as Hess Rise was passing under the equatorial high-productivity zone and was subsiding from shallow to intermediate depths. The association of volcanogenic components with organic-carbon-rich strata on Hess Rise in the Mid-Pacific Mountains is striking and suggests that there was a coincidence of mid-plate volcanic activity and the production and accumulation of organic matter at intermediate water depths in the tropical Pacific Ocean during the middle Cretaceous. Pyrolysis assays and analyses of extractable hydrocarbons indicate that the organic matter in the limestone on Hess Rise is composed mainly of lipid-rich kerogen derived from aquatic marine organisms and bacteria. Limestones from the Mid-Pacific Mountains generally contain low ratios of pyrolytic hydrocarbons to organic carbon and low hydrogen indices, suggesting that the organic matter may contain a significant proportion of land-derived material, possibly derived from numerous volcanic islands that must have existed before the area subsided. The organic carbon in all samples analyzed is isotopically light (d13C -24 to -29 per mil) relative to most modern rine organic carbon, and the lightest carbon is also the most lipid-rich. There is a positive linear correlation between sulfur and organic carbon in samples from Hess Rise and from the Mid-Pacific Mountains. The slopes and intercepts of C-S regression lines however, are different for each site and all are different from regression lines for samples from modern anoxic marine sediments and from Black Sea cores. The organic-carbon-rich limestones on Hess Rise, the Mid-Pacific Mountains, and other plateaus and seamounts in the Pacific Ocean are not synchronous but do occur within the same general middle Cretaceous time period as organic-carbon-rich lithofacies elsewhere in the world ocean, particularly in the Atlantic Ocean. Strata of equivalent age in the deep basins of the Pacific Ocean are not rich in organic carbon, and were deposited in oxygenated environments. This observation, together with the evidence that the plateau sites were considerably shallower and closse to the equator during the middle Creataceous suggests that local tectonic and hydrographic conditions may have resulted in high surface-water productivity and the preservation of organic matter in an oxygen-deficient environment where an expanded mid-water oxygen minimum developed and impinged on elevated platforms and seamounts.

Sedimentology and stable isotope record of DSDP Hole 68-503B

Samples from the upper portion of a cyclic pelagic carbonate sediment sequence in Deep-Sea Drilling Project (DSDP) hole 503B (4.0°N, 95.6°W) are the first group to be analyzed for paleoceanographic and paleoclimatic proxy-indicators of ice volume, deep ocean and surface water circulation, and atmospheric circulation in order to resolve the complex origin of the cyclicity. Temporal resolution is taken from the delta18O time scale, most other parameters are calculated in terms of their mass flux to the seafloor. CaCO3 percent in the sediments fluctuates in the well-known Pacific pattern and is higher during glacial times. The fluxes of opal and organic carbon have patterns similar to each other and show a variability of a factor of 2.5 to 4. The longer organic carbon record shows flux maxima during both glacial and interglacial times. The accumulation patterns of both opal and organic carbon suggest that the variability in surface water productivity and/or seafloor preservation of those materials is not simply correlated to glacial or interglacial periods. Eolian dust fluxes are greater during interglacial periods by factors of 2 to 5, indicating that eolian source regions in central and northern South America were more arid during interglacial periods. The record of eolian grain size provides a semiquantitative estimation of the intensity of the transporting winds. The eolian data suggest more intense atmospheric circulation during interglacial periods, opposite to the anticipated results. We interpret this observation as recording the southerly shift of the intertropical convergence zone to the latitude of hole 503B during glaciations.

Distribution and abundance of calcareous-nannoplankton species in the Cretaceous sediments of DSDP Leg 62 Holes

Cretaceous sediments were recovered at all four sites (Sites 463-466) of the central North Pacific drilled during Leg 62 of the Deep Sea Drilling Project. One of the objectives was to get more information about the development of ocean plankton communities and early evolution of planktonic groups of the Mesozoic. In this article, the Cretaceous calcareous nannofossils from two areas of the central North Pacific (Mid-Pacific Mountains and Hess Rise) are listed and discussed. (The Cenozoic calcareous nannofossils are discussed by R. Schmidt 1981). Coring was continuous at all sites. Mesozoic calcareous nannoplankton assemblages range on the Mid-pacific Mountains from Barremian to Early Maastrichtian, and on Hess Rise from Albian to Late Maastrichtian. (No calcareous nannofossils older than Barremian or Albian respectively were found).

Quaternary carbonate and stable isotope record of ODP Holes 133-817A and 133-818B

The Quaternary history of metastable CaCO3 input and preservation within Antarctic Intermediate Water (AAIW) was examined by studying sediments from ODP Holes 818B (745 mbsl) and 817A (1015 mbsl) drilled in the Townsville Trough on the southern slope of the Queensland Plateau. These sites lie within the core of modern AAIW, and near the aragonite saturation depth (~1000 m). Thus, they are well positioned to monitor chemical changes that may have occurred within this watermass during the past 1.6 m.y. The percent of fine aragonite content, percent of fine magnesian calcite content, and percent of whole pteropods (>355 µm) were used to separate the fine aragonite input signal from the CaCO3 preservation signal. Stable d18O and d13C isotopic ratios were determined for the planktonic foraminifer Globigerinoides sacculifer and, in Hole 818B, for the benthic foraminifer Cibicidoides spp. to establish the oxygen isotope stratigraphy and to study the relationship between intermediate and shallow water d13C of Sum CO2 and the relationship between benthic foraminiferal d13C and CaCO3 preservation within intermediate waters of the Townsville Trough. Data were converted from depth to age using oxygen isotope stratigraphy, nannostratigraphy, and foraminiferal biostratigraphy. Several long hiatuses and the absence of magnetostratigraphy did not permit time series analysis. The principal results of the CaCO3 preservation study include the following (1) a general increase in CaCO3 preservation between 0.9 and 1.6 Ma; (2) a CaCO3 dissolution maximum near 0.9 Ma, primarily expressed in the Hole 818B fine aragonite record; (3) an abrupt and permanent increase of fine aragonite content between 0.86 and 0.875 Ma in both Holes 818B and 817A probably reflecting a dramatic increase of fine carbonate sediment production on the Queensland Plateau; (4) an improvement in CaCO3 preservation near 0.87 Ma, which accompanied the increase of sediment input, indicated by the first appearance of whole pteropods in the deeper Hole 817A and a "spike" in the percent whole pteropods in Hole 818B; (5) a period of strong CaCO3 dissolution during the mid-Brunhes Chron from 0.36 to 0.41 Ma; and (6) a complex CaCO3 preservation pattern between 0.36 Ma and the present characterized by a general increase in CaCO3 preservation through time with good preservation during interglacial stages and poor preservation during glacial stages. The long-term aragonite preservation histories for Holes 818B and 817A appear to be similar in general shape, although different in detail, to CaCO3 preservation records from the deep Indian and central equatorial Pacific oceans as well as from intermediate water sites in the Bahamas and the Maldives. All of these areas have experienced CaCO3 dissolution at about 0.9 Ma and during the mid-Brunhes Chron. However, the late Quaternary (0 to 0.36 Ma) glacial to interglacial preservation pattern in Holes 818B and 817A is out of phase with CaCO3 preservation records for sediments deposited in Pacific deep and bottom waters. The sharp increase in bank production and export from the Queensland Plateau and the coincident improvement of CaCO3 preservation between 0.86 and 0.875 Ma may have been synchronous with the initiation of the Great Barrier Reef and roughly coincides with an increase in carbonate accumulation on the Bahama banks, in the western North Atlantic Ocean, and on Mururoa atoll, in the central South Pacific Ocean. The development of these reef systems during the middle Quaternary may be related to the transition in the frequency and amplitude of global sea level change from 41 k.y. low amplitude cycles prior to 0.9 Ma to 100 k.y. high amplitude cycles after 0.73 Ma. Carbon isotopic analyses show that benthic foraminiferal d13C values (Cibicidoides spp.) have been heavier than planktonic foraminiferal d13C values (G. sacculifer) throughout most of the last 0.54 m.y., which may indicate that 13C-enriched intermediate water (AAIW) occupied the Townsville Trough during much of the late Quaternary. Furthermore, both planktonic and benthic foraminiferal d13C values are often observed to be heaviest during interglacial to glacial transitions, and lightest during glacial to interglacial transitions. We suggest that this pattern is the result of changes in the preformed d13C of Sum CO2 of AAIW and may reflect changes in nutrient utilization by primary producers in Antarctic surface waters, changes in the d13C of upwelled Circumpolar Deep Water, or changes in the extent and/or temperature of equilibration between surface water and atmospheric CO2 within the Antarctic Polar Frontal Zone (the source area for AAIW). Finally, the poor correlation between percent of whole pteropods (aragonite preservation) and d13C of Cibicidoides spp. may be the result of a decoupling of d13C from CO2 due to the numerous and complex variables that combine to produce the preformed d13C of AAIW.

Chemical and isotopic compositions of rocks and minerals from the Franz Josef Land Archipelago

Rb, Sr, Sm, Nd, U, and Pb contents and Sr, Nd, and Pb isotopic composition were determined in tholeiite and subalkaline basalts (in both whole-rock samples and individual minerals) from the Franz Josef Land Archipelago. Isotopic data obtained for the Arctic basin are similar to those for islands from the Pacific, Atlantic, and Indian oceans. The assimilation of crustal (sedimentary) rocks by primary depleted material makes isochron determination of basalt age difficult or impossible. The subalkaline basalts (basaltic andesites) were presumably formed by the metasomatic introduction of incompatible elements in tholeiitie basalts and, only partially, through crustal contamination and fractional crystallization.

Composition of sediments and cores from DSDP Legs 54 and 61-63

The monograph gives results of studies of sediments and rocks collected from D/S Glomar Challenger in the Pacific Ocean. These studies have been based on the lithological facial analysis applied for the first time for identificating genesis of ocean sediments. These results include new ideas on formation of the Earth's sedimentary cover and can be used for constructing regional and global schemes of ocean paleogeography, reconstructing some structures, correlating sedimentation on continents and in oceans, estimating perspectives of oil- and gas-bearing deposits and ore formation. The monograph also gives the first petrographic classification of organic matter in black shales.

Calcium carbonate content and concentration of phosphorus components in sediments of ODP Leg 171B sites

Quantifying phosphorus (P) concentrations in marine sediments is necessary for constraining the oceanic record of phosphorus burial and helps to constrain P sedimentary geochemistry. To understand P geochemistry in the sediments, we must determine the geochemical forms of P as well as the transformations occurring between these P components with depth and age. Although several records now exist of P geochemistry in the western and eastern equatorial Pacific (Filippelli and Delaney, 1995, doi:10.2973/odp.proc.sr.138.144.1995; 1996, doi:10.1016/0016-7037(96)00042-7), the western equatorial Atlantic (Delaney and Anderson, 1997, doi:10.2973/odp.proc.sr.154.124.1997), the California Current (Delaney and Anderson, in press), and the Benguela Current (Anderson et al., 2001, doi:10.1029/2000GB001270), most of these are Neogene records. Relatively little data exist from sediments of the Paleogene or Cretaceous, time periods when carbon isotope records indicate major carbon shifts and when the nature of P geochemistry has not been well constrained. Samples from several sites at various water depths, oceanographic regions, and ages are needed to understand how P geochemistry and burial in sediments reflect ocean history. We determined P geochemistry and reactive P concentrations in Atlantic sediments of Eocene to Cretaceous age. These are the first records of P geochemistry with good age control from this period. Blake Nose sites are ideal for investigating P geochemistry, as the sediments are shallowly buried at a range of water depths and sedimentation rates. We determined P concentrations and geochemistry, along with calcium carbonate contents, in mid-Cretaceous to upper Eocene sediments drilled on Blake Nose (Ocean Drilling Program Leg 171B) in a depth transect of four sites (Sites 1052, 1051, 1050, and 1049; water depths: 1345, 1983, 2300, and 2656 m, respectively).

Anisotropy and acoustic properties at DSDP Holes 62-463 and 62-465A

Variations of acoustic properties within the sediment column may significantly affect the propagation of acoustic energy in the upper portion of the oceanic crust. Moreover, the acoustic properties of sediments reflect their mineral compositions, fabrics, and degrees of compaction and cementation. Hence, the physical properties of indurated deep-sea sediments are of considerable geophysical and geological interest. Chalks and limestones are particularly important because substantial accumulations of biogenic carbonates are generally present at the base of the deep-sea sediment column, and high-standing features such as Hess Rise are capped by calcareous deposits. This paper constitutes a preliminary report of the compressional-wave velocities and densities of 31 indurated calcareous sediment samples recovered at DSDP Sites 463 and 465, in the Mid-Pacific Mountains and on Hess Rise, respectively. The sample set includes nine pairs of samples in which velocities were measured parallel and perpendicular to bedding to determine the velocity anisotropy of the sediment. This research is part of an ongoing study of the seismic properties of indurated deep-sea carbonates.

Preliminary ichthyolith biostratigraphy and age profiles at DSDP Sites 91-595 and 91-596

Seventy meters of Cenozoic and Mesozoic pelagic clay cored at DSDP Sites 595 and 596 provide the basis for a preliminary analysis of ichthyolith biostratigraphy in the southwest Pacific. A most likely order of the more reliable ichthyolith events is compared with a synthesis of ichthyolith biostratigraphy in the North Pacific and with dated composite ranges. The resultant preliminary ichthyolith stratigraphy suggests that the Cenozoic is represented by the upper 20 m at Site 596 and 16 to 22 m at Site 595. Mixing of taxa precludes a clear recognition of the Cretaceous/Tertiary boundary at Site 595. The occurrence of 13 newly described subtypes is recorded in Mesozoic sediments at Sites 595 and 596. These new subtypes and previously described Mesozoic forms may be useful for recognizing Mesozoic subdivisions when their occurrences in sequences dated by other microfossils are investigated.

Age models and foraminifer isotopes of four ODP Leg 202 sites

We present benthic isotope stratigraphies for Sites 1236, 1237, 1239, and 1241 that span the late Miocene-Pliocene time interval from 6 to 2.4 Ma. Orbitally tuned timescales were generated for Sites 1237 and 1241 by correlating the high-frequency variations in gamma ray attenuation density, percent sand of the carbonate fraction, and benthic d13C to variations in Earth's orbital parameters. The astronomical timescales for Sites 1237 and 1241 are in agreement with the one from Atlantic Site 925/926 (Ocean Drilling Program Leg 154). The comparison of benthic d18O and d13C records from the east Pacific sites and Atlantic Site 925/926 revealed a surprising clarity of the "41-k.y. signal" in d13C records and a remarkably good correlation between their d13C records. This suggests that the late Miocene-Pliocene amplitudes of obliquity-related d13C cycles reflect a magnitude of global response often larger than that provided by obliquity-related d18O cycles. At Site 1237, the orbitally derived ages of Pliocene magnetic reversal boundaries between the base of Réunion and the top of Thvera confirm astronomical datings of the generally accepted ATNTS2004 timescale, except for the top of Kaena and the base of Sidufjall. Our astronomical age for the top of Kaena is about one obliquity cycle older. The base of Sidufjall appears to be about one precession cycle younger. The age models of Sites 1236 and 1239 were established by correlating their benthic d18O and d13C records directly to the orbitally tuned isotope record of Site 1241.

Strontium isotope data from DSDP Sites 32-305 and 62-463 and ODP Sites 113-689 and 113-690

Fluctuations in oxygen (d18O) and carbon (d13C) isotope values of benthic foraminiferal calcite from the tropical Pacific and Southern Oceans indicate rapid reversals in the dominant mode and direction of the thermohaline circulation during a 1 m.y. interval (71-70 Ma) in the Maastrichtian. At the onset of this change, benthic foraminiferal d18O values increased and were highest in low-latitude Pacific Ocean waters, whereas benthic and planktic foraminiferal d13C values decreased and benthic values were lowest in the Southern Ocean. Subsequently, benthic foraminiferal d18O values in the Indo-Pacific decreased, and benthic and planktic d13C values increased globally. These isotopic patterns suggest that cool intermediate-depth waters, derived from high-latitude regions, penetrated temporarily to the tropics. The low benthic d13C values at the Southern Ocean sites, however, suggest that these cool waters may have been derived from high northern rather than high southern latitudes. Correlation with eustatic sea-level curves suggests that sea-level change was the most likely mechanism to change the circulation and/or source(s) of intermediate-depth waters. We thus propose that oceanic circulation during the latest Cretaceous was vigorous and that competing sources of intermediate- and deep-water formation, linked to changes in climate and sea level, may have alternated in importance.

Calcareous nannofossils from the Sumisu Rift and Izu-Bonin Forearc Basin

During Ocean Drilling Program Leg 126, we recovered three expanded Pleistocene sections from the active backarc rift (Sumisu Rift) and three expanded Oligocene-Miocene sections from the forearc basin of the Izu-Bonin volcanic island arc. Quantitative analysis of the Pleistocene nannofossils revealed five major assemblages between 0 and LO Ma: Assemblage 1 (Holocene-0.085 Ma) contains dominant Emiliania huxleyi; Assemblage 2 (ca. 0.085-0.275 Ma) contains dominant small Gephyrocapsa and common E. huxleyi and Gephyrocapsa oceanica; Assemblage 3 (ca. 0.275-0.6 Ma) contains dominant Gephyrocapsa caribbeanica; Assemblage 4 (ca. 0.6-0.9 Ma) contains a peak abundance of small Gephyrocapsa in the middle part, and dominant occurrences of two types of G. caribbeanica in the lower and upper parts; and Assemblage 5 (ca. 0.9-1.0 Ma) contains dominant small Gephyrocapsa and common G. caribbeanica and Reticulofenestra asanoi. These assemblages are largely synchronous with similar assemblages recognized from tropical and subtropical regions, and can be used for finer subdivision of the Pleistocene than that based on standard Pleistocene nannofossil datums. The Oligocene-Miocene sections contain several hiatuses: up to 3 m.y. may be missing from the uppermost Oligocene (Zone CP19) at Sites 792 and 793; all of Zone CN2 is missing at Sites 792 and 793; part of Zone CN3 and all of Zone CN4 are missing at Site 792. Biochronology of several nannofossil datums at Leg 126 sites indicate that Sphenolithus distentus, Sphenolithus ciperoensis, Cyclicargolithus floridanus, and Discoaster kugleri have diachronous occurrences compared with other sites in the western Pacific Ocean and Philippine Sea.

Age model and Plio-Pleistocene benthic stable oxygen isotope ratios of ODP Site 184-1143 in the Sourh China Sea

Based on benthic foraminiferal delta18O from ODP Site 1143, a 5-Myr astronomical timescale for the West Pacific Plio-Pleistocene was established using an automatic orbital tuning method. The tuned Brunhes/Matuyama paleomagnetic polarity reversal age agrees well with the previously published age of 0.78 Ma. The tuned ages for several planktonic foraminifer bio-events also agree well with published dates, and new ages for some other bio-events in the South China Sea were also estimated. The benthic delta18O from Site 1143 is highly coherent with the Earth's orbit (ETP) both at the obliquity and precession bands for the last 5 Myr, and at the eccentricity band for the last 2 Myr. In general, the 41-kyr cycle was dominant through the Plio-Pleistocene although the 23-kyr cycle was also very strong. The 100-kyr cycle became dominant only during the last 1 Myr. A comparison of the benthic delta18O between the Atlantic (ODP 659) and the East and West Pacific (846 and 1143) reveals that the Atlantic-Pacific benthic oxygen isotope difference ratio (Delta delta18OAtl-Pac) displays an increasing trend in three time intervals: 3.6-2.7 Ma, 2.7-2.1 Ma and 1.5-0.25 Ma. Each of the intervals begins with a rapid negative shift in Delta delta18OAtl-Pac, followed by a long period with an increasing trend, corresponding to the growth of the Northern Hemisphere ice sheet. This means that all three intervals of ice sheet growth in the Northern Hemisphere were accompanied at the beginning by a rapid relative warming of deep water in the Atlantic as compared to that of the Pacific, followed by its gradual relative cooling. This general trend, superimposed on the frequent fluctuations with glacial cycles, should yield insights into the processes leading to the boreal glaciation. Cross-spectral analyses of the Delta delta18OAtl-Pac with the Earth's orbit suggests that after the initiation of Northern Hemisphere glaciation at about 2.5 Ma, obliquity rather than precession had become the dominant force controlling the vertical structure or thermohaline circulation in the paleo-ocean.