Calcareous nannofossils of ODP sites 177-1088 and 177-1090

During ODP Leg 177, a Miocene to Pliocene calcareous nannofossil record was recovered at Sites 1088 and 1090. Site 1088, located at 41°8'S, shows a continuous middle-upper Miocene to Pliocene carbonate sequence that was deposited at relatively high sedimentation rates (Shipboard Scientific Party, 1999a, doi:10.2973/odp.proc.ir.177.103.1999). Moreover, Site 1088 proved suitable for calcareous nannofossil analysis as a means to improve the biostratigraphy at this southern latitude. Site 1090 was drilled at 42°54'S; a tephra layer marks a significant disconformity at the Miocene/Pliocene boundary of this sequence (Shipboard Scientific Party, 1999b, doi:10.2973/odp.proc.ir.177.105.1999). Although nannofossil assemblages are poorly preserved at this site (Shipboard Scientific Party, 1999b), they may help in determining the age of the disconformity and its paleoceanographic significance.

(Table 1) Coarse-fraction (0.1 to 0.05 mm) mineralogy at DSDP Leg 55 Holes

This chapter was previously intended to trace volcanic episodes through the Neogene and Pleistocene geological history recorded in the sedimentary sections drilled on the Emperor seamounts. Drilling disturbance, poor core recovery, and incomplete stratigraphic sections recovered from the seamounts have frustrated that plan, however. Moreover, the Leg 55 sedimentologists found in their smear-slide studies that transported island-arc tephra is scarce in the sediments, if present at all. So we have restricted our objective to description of the volcaniclastic admixture in sediments, as determined by mineralogical and geochemical data. We studied geochemistry of bulk samples (see Murdmaa et al., 1980), coarse-fraction mineralogy, and additional smear slides. The results obtained, however, do not tell much more about the volcaniclastic matter than did shipboard core descriptions.

Tephrostratigraphy and radiometric dating of authigenic carbonates

At convergent margins, fluids rise through the forearc in response to consolidation of the upper plate and dewatering of the subducting plate, and produce various cold-seep-related features on the seafloor (mud diapirs, mud mounds). At the Central American forearc, authigenic carbonates precipitated from rising fluids within such structures during active venting while typical mixed-mud sediments were ejected onto the surrounding seafloor where they became intercalated with normal pelagic background sediments, indicating that mud mounds evolved unsteadily through alternating active and inactive phases. Intercalated regional ash layers from Plinian eruptions at the Central American volcanic arc provide time marks that constrain the ages of mud ejection activity. U/Th dating of drill core samples of authigenic carbonate caps of mud mounds yields ages agreeing well with those constrained by ash layers and showing that carbonate caps grow inward rather than outward during active venting. Both dating approaches show that offshore Nicaragua and Costa Rica (1) active and inactive phases can occur simultaneously at neighboring mounds, (2) mounds along the forearc have individual histories of activity, but there are distinct time intervals when nearly all mounds have been active or inactive, (3) lifetimes of mounds reach several hundred thousand years, and (4) highly active periods last 10-50 k.y. with intervening periods of >10 k.y. of relative quiescence.

Volcanic ash from the Peru continental margin

During Leg 112 off Peru, volcanic material was recorded from middle Eocene to Holocene time. The petrographical and chemical composition of tephra is consistent with an origin from the Andean volcanic arc. The amount and thickness of ash layers provide valuable evidence for explosive volcanic episodicity. The first indication of volcanism was found in mid-Eocene sediments. Three volcanic pulses date from Miocene time. Two intense episodes took place in upper Pliocene and from Pleistocene to Holocene time. Pliocene-Pleistocene tephra are restricted to the southern upper-slope and shelf sites, indicating a removal of the volcanic arc and the extinction of the northern Peru volcanoes. The Cenozoic tectonic phases of the Andean margin may be correlated with the Leg 112 volcanic records. The explosive supply of evolved magmatic products succeeded the Incaic and Quechua tectonic phases. Acidic glasses are related to both andesitic and shoshonitic series. The calc-alkaline factor (CAF) of these glasses exhibited moderate magmatic variations during middle and late Miocene time. A dramatic change occurred during the Pliocene-Pleistocene, reflected in a strong CAF increase and the appearance of potassium-rich evolved shoshonitic glasses. This took place when the Nazca Ridge subduction began. This change in the magma genesis and/or differentiation conditions is probably related to thickening of the upper continental plate and to a new configuration of the Benioff Zone.

Geochemistry of Cenozoic ash layers from the Kerguelen Plateau

Geochemical investigations were conducted on 10 discrete ash layers and 22 samples of dispersed ash accumulations from Sites 747, 749, and 751 of Ocean Drilling Program (ODP) Leg 120 to the Kerguelen Plateau in the southern Indian Ocean. The chemical data obtained from some 400 single-grain glass analyses allow the characterization of two rock series. The first consists of transitional to alkali basalts; the second, mainly of trachytes with subordinated rhyolites, all reflecting the characteristic magmatological evolution of the Kerguelen Plateau as a hotspot-related volcanism. Chemical correlation with possible source areas indicates that the ashes were most probably erupted from the Kerguelen Islands. The investigated ash layers clearly reflect the Oligocene to Quaternary changes in the composition of the volcanic material recorded from the Kerguelen Islands. In addition to the Kerguelen Islands, Heard Island, Crozet Island, and other sources may have contributed to deposition of the tephras. Pleistocene tephras of "exotic" calc-alkaline composition are most probably derived from enhanced magmatic activity during that time span at the South Sandwich island arc. When using data obtained from tephras of the ODP Leg 119 Kerguelen sites, several eruptive periods can be correlated through the composition of the deposited ashes. Some of them are widely distributed over the Kerguelen Plateau and are seen as a first step toward a southern Indian Ocean tephrostratigraphy.

Pollen record from Muggesfelder See and Plussee in Germany

A basaltic tephra layer consisting of brownish-olive glass shards. and about 0.2 mm thick. was found in cores from four lakes in northwest Germany. According to pollen analysis it was deposited during the early Boreal period (corresponding to about 8700 BP). The petrographic properties. the geochemical composition and the age agree with those of the Saksunarvatn tephra. which was first found on the Faroe Islands. The position of the tephra layer in the pollen stratigraphy and in the absolute time-scale is discussed. Procedures for locating the tephra in other cores are suggested.

Geochemistry and petrographic characters at DSDP Leg 58 Holes

Refractive index and chemical composition were determined for glass shards contained in more than 100 tephra layers in DSDP Leg 58 sediment cores collected in the Shikoku Basin, North Philippine Sea. The refractive index is consistent with chemical composition. Refractive index and total iron show a linear relationship. Tephra in Pleistocene and Pliocene sediments is mostly rhyolitic and dacitic (non-alkali), whereas tephra in the Miocene shows wide composition variations in the eastern part of the basin. Basaltic tephra is recognized in Miocene sediments at Sites 443 and 444, but not at Site 442, west of the other two sites. This indicates that the basaltic tephra came from eruption relatively close to those drill sites (perhaps the Kinan Seamounts and the Shichito-Iwo Jima volcanic arc), although the exact source has not been identified.

Occurrence of interlayers in sediment cores collected in the East Indian Ocean

In the East Indian Ocean direct contribution of land volcanism to sedimentation appears as interlayers of tephra and tuffaceous sediments, pumice fragments, and dispersed volcanoclastic materials of silty grain size. Similarity of distribution of tephra, tuffaceous sediments, Ethmodiscus ooze, and turbidites in the Pleistocene section results from deposition of all these materials under controll of a single factor, namely synchronous redistribution owing to seismic activity on the ocean floor and on the Sunda Islands. Burial of layers of oxidized deposits and formation of iron-manganese nodules is at least partly related to global climate cooling and to circulation of ocean waters.

Mineral composition, major element oxides and trace element concentration of sediments from ODP Leg 180 sites

Middle Miocene to Holocene fine-grained argillaceous sediments (clays, claystones/muds, and mudstones), which volumetrically dominated the sediment recovery in the Woodlark Basin during Leg 180, were chemically analyzed for major elements, trace elements, and some rare earth elements by X-ray fluorescence. Selected samples also underwent X-ray diffraction (XRD) analysis for mineral determination. The results shed light on sediment provenance when combined with shipboard sediment descriptions, smear slide study, and XRD. The oldest sediments recovered (Site 1108) of middle-late Miocene age include volcanogenic muds with distinctive high MgO and K2O, indicative of a relatively basic calc-alkaline source related to an inferred Miocene forearc succession. The forearc basement, composed of diabase and basalt, was locally exposed (Site 1109) and eroded in the late Miocene (<5.4-9.93 Ma), giving rise to fluvial conglomerates (Sites 1109, 1115, and 1118). Chemically distinctive fine-grained claystones and siltstones (with relatively high Ti, low K) are compatible with derivation from tropically weathered basic igneous rocks, correlated with the Paleogene Papuan ophiolite. Overlying latest Miocene-Pleistocene fine-grained sediments throughout the Woodlark Basin were partly derived from calc-alkaline volcanic sources. However, relatively high abundances of Al2O3 and related element oxides (K2O and Na2O) and trace elements (e.g., Rb and Y) reflect an additional terrigenous input throughout the basin, correlated with pelitic metamorphic rocks exposed on Papua New Guinea and adjacent areas. In addition, sporadic high abundances of Cr and Ni, some other trace metals, and related minerals (talc, crysotile, and chlorite) reflect input from an ophiolitic terrain dominated by ultramafic rocks, correlated with the Paleogene Papuan ophiolite. The source areas possibly included serpentinized ultramafic ophiolitic rocks exposed in the Papua New Guinea interior highlands. Chemical evidence further indicates that fine-grained terrigenous sediment reached the Woodlark Basin throughout its entire late Miocene-Holocene history. Distinctive high-K volcanogenic muds rich in tephra and volcanic ash layers that appear at <2.3 Ma (Sites 1109 and 1115) are indicative of high-K calc-alkaline volcanic centers, possibly located in the Dawson Strait, Moresby Strait, or Dobu Seamount area. Chemical diagenesis of fine-grained sediments within the Woodlark Basin is reflected in clay neomorphism and localized formation of minerals including dolomite, ankerite, and zeolite but has had little effect on the bulk chemical composition of most samples.

Age models of northern North Atlantic sediment cores

Benthic foraminiferal stable isotope records from four high-resolution sediment cores, forming a depth transect between 1237 m and 2303 m on the South Iceland Rise, have been used to reconstruct intermediate and deep water paleoceanographic changes in the northern North Atlantic during the last 21 ka (spanning Termination I and the Holocene). Typically, a sampling resolution of ~100 years is attained. Deglacial core chronologies are accurately tied to North Greenland Ice Core Project (NGRIP) ice core records through the correlation of tephra layers and changes in the percent abundance of Neogloboquadrina pachyderma (sinistral) with transitions in NGRIP. The evolution from the glacial mode of circulation to the present regime is punctuated by two periods with low benthic d13C and d18O values, which do not lie on glacial or Holocene water mass mixing lines. These periods correlate with the late Younger Dryas/Early Holocene (11.5-12.2 ka) and Heinrich Stadial 1 (14.7-16.8 ka) during which time freshwater input and sea-ice formation led to brine rejection both locally and as an overflow exported from the Nordic seas into the northern North Atlantic, as earlier reported by Meland et al. (2008). The export of brine with low ?13C values from the Nordic seas complicates traditional interpretations of low d13C values during the deglaciation as incursions of southern sourced water, although the spatial extent of this brine is uncertain. The records also reveal that the onset of the Younger Dryas was accompanied by an abrupt and transient (~200-300 year duration) decrease in the ventilation of the northern North Atlantic. During the Holocene, Iceland-Scotland Overflow Water only reached its modern flow strength and/or depth over the South Iceland Rise by 7-8 ka, in parallel with surface ocean reorganizations and a cessation in deglacial meltwater input to the North Atlantic.

Diatom biostratigraphy, Argon ages and chronology of ODP Hole 183-1138A

A thick Neogene section was recovered in the upper ~300 m of Ocean Drilling Program Hole 1138A, drilled on the Central Kerguelen Plateau in the Indian sector of the Southern Ocean. Sediment lithologies consist primarily of mixed carbonate and biosiliceous clays and oozes, with several thin (1-3 cm) tephra horizons. The tephras are glass rich, well sorted, and dominantly trachytic to rhyolitic in composition. Volcaniclastic material in these horizons is interpreted to have originated from Heard Island, 180 km northwest of Site 1138, and was likely emplaced through both primary ash fall and turbiditic, submarine flows. A Neogene age-depth model for Hole 1138A is constructed primarily from 36 diatom biostratigraphic datums. Nannofossil and planktonic foraminifer biostratigraphy provides supporting age information. Additionally, four high-precision 40Ar-39Ar ages are derived from ash and tephra horizons, and these radiometric ages are in close agreement with the biostratigraphic ages. The integrated age-depth model reveals a reasonably complete lower Miocene to upper Pleistocene section in Hole 1138A, with the exception of a ~1-m.y. hiatus at the Miocene/Pliocene boundary. Another possible hiatus is also identified at the Oligocene/Miocene boundary. High Neogene sedimentation rates and the presence of both calcareous and siliceous microfossils, combined with datable tephra horizons, establish Site 1138 as a suitable target for future drilling legs with paleoceanographic objectives. This report also proposes two new diatom species, Fragilariopsis heardensis and Azpeitia harwoodii, from Pliocene strata of Hole 1138A.