Mineralogy of southern Kerguelen Plateau sediments

Kaolinite, goethite, minor hematite, and gibbsite were found in fluvial upper Lower Cretaceous basal sediment from the Southern Kerguelen Plateau, Sites 748 and 750, 55°S latitude. This mineral assemblage, derived from the weathering of basalt, indicates near-tropical weathering conditions with high orographic rainfall, at least 100 cm per year. The climate deteriorated by the Turonian or Coniacian, as indicated by the decline in kaolinite content of this sediment. The Upper Cretaceous sediment at Site 748 consists of 200 m of millimeter-laminated, sparsely fossiliferous, wood-bearing glauconitic siltstone and clay stone with siderite concretions deposited on a shelf below wave base. Some graded and cross beds indicate that storms swept over the shelf and reworked the sediment. Overlying this unit is 300 m of intermittently partly silicified, bryozoan-inoceramid-echinoderm-rich glauconitic packstones, grainstones, and wackestones. The dominant clay mineral in both units is identical to the mineral composition of the glauconite pellets: randomly interstratified smectite-mica. The clay fraction has a higher percent of expandable layers than the mineral of the glauconite pellets, and the clay of the underlying subunit has a higher percentage of expandable layers than the clay of the carbonate subunit. Potassium levels mirror these mineral variations, with higher K levels in minerals that have a lower percentage of expandable layers. The decrease in expandability of the mineral in the upper subunit is attributed to diagenesis, the result of higher porosity.

Sea surface temperature reconstruction for the middle Miocene Southern Ocean

Magnesium/calcium data from Southern Ocean planktonic foraminifera demonstrate that high-latitude (~55°S) southwest Pacific sea surface temperatures (SSTs) cooled 6° to 7°C during the middle Miocene climate transition (14.2 to 13.8 million years ago). Stepwise surface cooling is paced by eccentricity forcing and precedes Antarctic cryosphere expansion by ~60 thousand years, suggesting the involvement of additional feedbacks during this interval of inferred low-atmospheric partial pressure of CO2 (pCO2). Comparing SSTs and global carbon cycling proxies challenges the notion that episodic pCO2 drawdown drove this major Cenozoic climate transition. SST, salinity, and ice-volume trends suggest instead that orbitally paced ocean circulation changes altered meridional heat/vapor transport, triggering ice growth and global cooling.

Quartz and opaline silica in ocean sediments

A new method of quantitative analysis of quartz and opal in bottom sediments is developed. It is based on the study of sediment samples in form of suspensions in petrolatum where potassium rhodanate is added as an internal standard.

Neogene planktonic foraminifers from Wombat and Exmouth Plateau

Diverse, warm-water planktonic foraminiferal faunas prevailed on the Wombat and Exmouth plateaus during the Neogene, in spite of the northward drift of Australia across 10° to 15° latitude since the early Miocene. Invasions of cool-water species occurred during periods of global cooling in the late middle Miocene, late Miocene, and Pleistocene, and reflect periods of increased northward transport of cool surface water, probably via the West Australian Current. The sedimentary record of the Neogene on Wombat and Exmouth Plateau is interrupted by two hiatuses (lower Miocene, Zone N5, and upper middle to upper Miocene, Zones N15-N17), and one redeposited section of upper Miocene to uppermost Pliocene sediments. Mechanical erosion or nondeposition by increased deep-water flow or tilting and uplift of Wombat and Exmouth plateaus, resulting in sediment shedding, are the most likely explanations for these Miocene hiatuses, but which of these processes were actually operative on the Wombat and Exmouth plateaus is uncertain. The redeposited section of upper Miocene to uppermost Pliocene sediments in Hole 761B, however, certainly reflects a latest Pliocene period of uplift and tilting of the Wombat Plateau. An important finding was the occurrence of Zone N15-correlative sediments in Hole 762B without any representative of Neogloboquadrina. Similar findings in Java and Jamaica indicate that the earliest spreading of Neogloboquadrina acostaensis in the tropical region resulted from migration. The evolution of this species, therefore, must have taken place in higher latitudes. I suggest that Neogloboquadrina acostaensis evolved from Neogloboquadrina atlantica in the North Atlantic within Zone NN9, but how and where in the region this speciation took place is still uncertain

Trace element geochemistry of zircons frim in situ ocean lithoshere

We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a TiO2 ~ 0.7, a SiO2 ~ 1.0, pressure ~ 2 kbar); intra-sample variation is typically ~60-15°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680-740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La)N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.