Pacific Deep Sea Drilling Project Research Articles

Pliocene climate change of the Southwest Pacific and the impact of ocean gateways

The transition from the early Pliocene “Warmhouse” towards the present “Icehouse” climate and the role of Gateway dynamics are intensively debated. Both, the constrictions of the Central American Seaway and the Indonesian Gateway affected ocean circulation and climate during the Pliocene epoch. Here, we use combined δ18O and Mg/Ca ratios of planktonic foraminifera (marine protozoa) from surface and subsurface levels to reconstruct the thermal structure and changes in salinities from the Southwest Pacific Deep Sea Drilling Project (DSDP) Site 590B from 6.5 to 2.5Ma. Our data suggest a gradual cooling of ~2°C and freshening of the sea surface during ~4.6–4Ma with an increased meridional temperature gradient between the West Pacific Warm Pool and the Southwest Pacific when the closing of the Central American Seaway reached a critical threshold. After ~3.5Ma, the restricted Indonesian Gateway might have amplified the East Australian Current, allowing enhanced heat transport towards the Southwest Pacific with reduced meridional temperature gradients when the global climate gradually cooled. At the same time our data suggest a cooling and freshening of Subantarctic Mode Water (SAMW) or/and an increased northward flow of SAMW towards Site 590B, possibly a first step towards the present Antarctic Frontal System.

N-type magnetism at cryogenic temperatures in oceanic basalt

Here we report on a self-reversal of magnetization at cryogenic temperatures (25–60 K) due to N-type thermomagnetic behavior in samples of Cretaceous mid-oceanic ridge basalt recovered from Pacific Deep Sea Drilling Project Site 166. The magnetic carrier is highly oxidized single to pseudo-single domain titanomaghemite with a composition (0.57 ≤ x ≤ 0.66, 0.8 ≤ z ≤ 0.9) consistent with model predictions of ionic reordering. The observation of N-type magnetization provides further insight into the compositional field in which ionic reordering and self-reversal can occur in oceanic basalt. Specifically, our observations support the original models of Verhoogen [Verhoogen, J., 1956. Ionic reordering and self-reversal of magnetization in impure magnetites. J. Geophys. Res. 61, 201–209; Verhoogen, J., 1962. Oxidation of iron–titanium oxides in igneous rocks. J. Geol. 70, 168–181] and O’Reilly and Banerjee [O’Reilly, W., Banerjee, S.K., 1966. Oxidation of titanomagnetites and self-reversal. Nature 211 (5044), 26–28] indicating that self-reversal above and below room temperature is limited to relatively high titanium contents ( x > 0.6) and high oxidation states.

The Cretaceous‐Tertiary extinction: Modeling carbon flux and ecological response

It is widely recognized that a significant negative excursion in carbon isotopic (δ13C) differences between planktic and benthic foraminiferal tests occurred at the Cretaceous‐Tertiary (K‐T) boundary. We applied parametric and nonparametric breakpoint tests and statistical comparisons of different recovery models to assess the timing and pattern of recovery from this negative excursion at South Atlantic Deep Sea Drilling Project (DSDP) Site 528 and equatorial Pacific DSDP Site 577. Our results indicate a two‐stage recovery with an initial recovery to an intermediate state of planktic‐to‐benthic δ13C differences followed by a discontinuous shift to a final state with planktic‐to‐benthic δ13C differences similar to preextinction values. The final discontinuous shift in both the Pacific and Atlantic Ocean sites occurred several million years after the K‐T collapse of planktic‐to‐benthic δ13C differences. Both the first and second stages of recovery are best described by damped exponential relaxations. The pattern and timing of this carbon cycle recovery may have been contingent on the occurrence of key biological events.

Benthic foraminiferal response to the emergence of the Isthmus of Panama and coincident paleoceanographic changes

Late Cenozoic benthic foraminiferal faunas from the Caribbean Deep Sea Drilling Project (DSDP) Site 502 (3052 m) and East Pacific DSDP Site 503 (3572 m) were analyzed to interpret bottom-water masses and paleoceanographic changes occurring as the Isthmus of Panama emerged. Major changes during the past 7 Myr occur at 6.7–6.2, 3.4, 2.0, and 1.1 Ma in the Caribbean and 6.7–6.4, 4.0–3.2, 2.1, 1.4, and 0.7 Ma in the Pacific. Prior to 6.7 Ma, benthic foraminiferal faunas at both sites indicate the presence of Antarctic Bottom Water (AABW). After 6.7 Ma benthic foraminiferal faunas indicate a shift to warmer water masses: North Atlantic Deep Water (NADW) in the Caribbean and Pacific Deep Water (PDW) in the Pacific. Flow of NADW may have continued across the rising sill between the Caribbean and Pacific until 5.6 Ma when the Pacific benthic foraminiferal faunas suggest a decrease in bottom-water temperatures. After 5.6 Ma deep-water to intermediate-water flow across the sill appears to have stopped as the bottom-water masses on either side of the sill diverge. The second change recorded by benthic foraminiferal faunas occurs at 3.4 Ma in the Caribbean and 4.0-3.2 Ma in the Pacific. At this time the Caribbean is flooded with cold AABW, which is either gradually warmed or is replaced by Glacial Bottom Water (GBW) at 2.0 Ma and by NADW at 1.1 Ma. These changes are related to global climatic events and to the depth of the sill between the Caribbean and Atlantic rather than the rising Isthmus of Panama. Benthic foraminiferal faunas at East Pacific Site 503 indicate a gradual change from cold PDW to warmer PDW between 4.0 and 3.2 Ma. The PDW is replaced by the warmer, poorly oxygenated PIW at 2.1 Ma. Although the PDW affects the faunas during colder intervals between 1.4 and 0.7 Ma, the PIW remains the principal bottom-water mass in the Guatemala Basin of the East Pacific.

Evolving Pliocene-Pleistocene climate: A North Pacific perspective

Siliceous microfauna in sediments from North Pacific Deep Sea Drilling Project sites record the response of the regional surface and near-surface waters to global climate change over the past 3.0 Ma. During the Late Pliocene (2.7–3.0 Ma), the northern subtropical and subpolar Pacific experienced warmer climate conditions than those of today. The first predominant polar assemblage maximum occurs in the North Pacific at ∼2.45 Ma signifying this region's response to the initiation of significant northern hemisphere glaciation. North Pacific faunal data show that Late Pliocene (1.8–2.5 Ma) glacial maxima were only about half to two-thirds as severe as those which occurred during the Late Pleistocene. Low faunal assemblage variability during a 200 ka interval in the latest Pliocene (1.6–1.82 Ma) in the North Pacific indicates minimal variations in the surface water properties and climate conditions most like those which existed just prior to 2.5 Ma. A distinct change in faunal assemblage registered in sediments from Site 580 between 1.2 and 1.2 Ma marks the first cooling in the North Pacific comparable in intensity to those of the Late Pleistocene. Since ∼1.26 Ma, high amplitude, high frequency variations in the North Pacific polar assemblage have continued to occur, providing further evidence that the interglacial/glacial extremes that characterize the Late Pleistocene have persisted since this time. The frequency distribution of North Pacific winter sea-surface temperature time series for the Late Pliocene and Early Pleistocene includes major concentrations of variance (power) centered at a frequency corresponding to a 41 ka period, with variance dominance of the temperature time series for the last 800 ka shifting to lower frequencies (∼110 ka period).

Eustatic, Volcanic, and Tectonic Contols on Redeposition of Shallow-Water Carbonate: A Reanalysis of Pacific DSDP Examples: ABSTRACT

On the basis of comparisons between eustatic sea level curves and occurrences of displaced shallow marine carbonate sediment in deep water Pacific DSDP (Deep Sea Drilling Project) cores, several previous workers have proposed that most displaced neritic carbonate is redeposited in the deep sea during eustatic sea level lowstands in a manner similar to siliciclastic sediment. However, reanalysis of these same DSDP data indicates a much more complex set of controls on redeposition including eustatic, volcanic, and tectonic effects. Of the 43 occurrences identified in 20 DSDP sites, eight (including all occurrences of true allodapic calcarenites) were deposited either during eustatic highstands or immediately after cessation of island-building volcanism; establishment of a shallow-water source area probably represents the dominant control on this type of deposition. Twenty-three other occurrences contain volcanic detritus mixed with minor amounts of reworked carbonate. Of these occurrences eight contain volcaniclastic sediment and exhibit no clear correlation with sea level, indicating probable redeposition of shelf carbonate during volcanic events as a result of either gravitational or seismic instability of the shelf. In contrast, the 15 other mixed volcanic/carbonate occurrences represent lowstand deposit and contain epiclastic volcanic detritus, suggesting lowstand erosion of mixed volcanic/carbonate shelf sediments. Five othermore » lowstand occurrences contain only reworked carbonate sediment and probably record either continued outer shelf/slope lowstand carbonate deposition or minor mechanical erosion of shelf carbonate. Six occurrences contain only reworked Inoceramus fragments, which are not necessarily indicative of a shelf source area.« less

Abyssal circulation and benthic foraminiferal changes near the Paleocene/Eocene boundary

A major change in Cenozoic deep‐sea benthic foraminifera occurred in the Atlantic, Indian, and Pacific oceans near the Paleocene/Eocene boundary. Benthic foraminiferal abundance changes began at about 61.5 Ma at Pacific Deep Sea Drilling Project (DSDP) Site 577. A major extinction event followed at 58–57 Ma (between Zones P6a and P6b), and a series of first appearances continued until circa 55.5 Ma (Zone P6c). These faunal changes occurred during a 6°C warming of Pacific bottom water and may indicate that the primary cause was changing temperature. Other potential causes of the faunal turnover include global changes in surface ocean productivity and changing bottom water source regions. Comparison of benthic and planktonic δ13C records requires no change in the ratio of oceanic phosphorous to carbon during the late Paleocene to early Eocene, which weakens the case for (but does not disprove) a change in surface ocean productivity at this time. Interbasinal comparisons of benthic foraminiferal δ13C records document that water with high δ13C values filled the Cape Basin during the late Paleocene and possibly the early Eocene (circa 61–57 Ma), but apparently did not extend into the western basins of the Atlantic. This pattern suggests a supply of Antarctic source water for the Cape Basin and possible tectonic isolation of the western Atlantic basins during at least part of the late Paleocene. Carbon isotope comparisons show that bottom water supply to the Cape Basin was reduced in the early Eocene. Eolian grain size data suggest that a decrease in zonal wind intensity occurred at the end of the Paleocene. These late Paleocene climatic changes (bottom water warming and decreased wind intensity) correspond with evidence for an important global tectonic reorganization and extensive subaerial volcanism, which may have contributed to climatic warming through increased supply of CO2.

Neogene Deep-Living Radiolarian Zonation and Its Uses: ABSTRACT

A Neogene radiolarian zonation using deep-living radiolarians has been developed. This zonation consists of 6 radiolarian zones with the nominant taxa of each zone first occurring at the base of its zone, and that nominant taxa running throughout its zone. These zones are the Eucyrtidium cienkowskii (22.5-19 Ma), Siphocampe arachnea (19-14 Ma), Oroscena with digitate spines (14-11 Ma), Botryostrobus bramlettei (11-6 Ma), Clathrocyclas bicornis (6-2.5 Ma), and Cycladophora davisiana davisiana (2.5-0 Ma) Zones. These zones were developed by using Antarctic, equatorial, and North Pacific Deep Sea Drilling Project holes, and are believed to be cosmopolitan zones due to their being based on deep-living radiolarians. These zones are dated by correlating the first occurrences of the nominant taxa with warm-water radiolarian datums that have been paleomagnetically dated. These zones are not designed for routine use, but rather for tying together other more provincial zonations, and for comparing oceanographic phenomena on global datum planes. To date, this zonation has been used to correlate Antarctic zonations intra- and inter-regionally and to study the evolution of radiolarian lineages. End_of_Article - Last_Page 243------------

Early to Middle Miocene Planktonic Foraminiferal Datum Levels of the Equatorial and Subtropical Pacific

Early to Middle Miocene Planktonic Foraminiferal Datum Levels of the Equatorial and Subtropical Pacific

Neogene Radiolarian Biostratigraphy and Paleo-Oceanography of North Boundary Currents: ABSTRACT

Deep Sea Drilling Project (DSDP) cores from offshore Japan and California, land-based sections from California, and Holocene sediment samples from surrounding areas have been used to develop radiolarian biostratigraphies and to interpret paleo-oceanographic conditions for the Neogene of the Japan Current (a western boundary current) and the California Current (an eastern boundary current). Our studies suggest that (1) the present-day characteristics of boundary currents (western being deeper, narrower, swifter, and less productive than eastern boundary currents) have varied greatly throughout the Neogene, but are tied one to another; and (2) both mid-latitude regions can be correlated biostratigraphically to low latitudes. The Japan region is most correlative to low-latit de radiolarian biostratigraphic zones. The developed biostratigraphy of the region and inferred paleo-oceanographic trends aid in correlation of the California region to low-latitude radiolarian biostratigraphies. Comparison of material from boundary-current regions with equatorial and subpolar North Pacific DSDP material suggests that tectonic events such as the uplift of Panama may be tied to the observed Neogene changes in these boundary-current regions. These studies have also been instrumental in indicating radiolarian taxa that may be used to suggest relative paleodepth of deposition and eutrophic conditions in these boundary-current regions. End_of_Article - Last_Page 514------------