167 Preliminary Report

Site 1022 (Proposed Site CA-2B)

Site 1022 is situated on the continental slope just south of the Mendocino Fracture Zone about 90 km from Cape Mendocino. The site is located at a depth of 1950 mbsl on a sliver of continental crust apparently carried seaward along the Mendocino Fracture Zone (Fig. 1). The main objective for drilling at Site 1022 was to obtain a record of surface- and deep-water properties from the Miocene through the Quaternary. Because this site is located only 90 km from the coast, the sediments should provide a good record of coastal upwelling. Site 1022 has important geochemical objectives. Organic carbon deposition should be relatively high, yet the geothermal gradient should be relatively low. This environment provides one of the end members needed to study preservation of bulk organic matter and of specific organic molecules. Geochemical indices of paleoproductivity and microfossil assemblages obtained from Site 1022 will also provide important data on nutrients carried by the California Current and upon how the carbon cycle can be affected by changes in productivity and climate.

The sedimentary sequence recovered from the three holes at Site 1022 consists of a 388-m-thick interval of Quaternary to early Pliocene (0.0-5.0 Ma) age. The sequence is divided into three parts: an upper nannofossil-dominated interval, a middle siliciclastic-clay-dominated interval, and a lower biosiliceous interval. These lithostratigraphic units are further divided into subunits based on minor changes in composition or diagenesis. The uppermost part of the sequence is marked by a 1- to 3-m-thick bed of glauconitic clay with silt and diatoms that represents the entire Quaternary. Below this layer, the upper part of the sequence is dominated by nannofossil clay to nannofossil ooze and diatom clay. This unit includes minor volcanic ash and barite-cemented horizons. The middle part of the sequence is dominated by clay to diatom clay with infrequent interbeds of clayey nannofossil ooze. Diatom content gradually increases from the middle part to the lower unit which is dominated by diatomite and clayey diatomite. A diagenetic boundary occurs at 360 mbsf where diatomite is transformed into interbedded siliceous mudstone and chert. Infrequent, decimeter-thick, dolostone beds occur in all parts of the sequence.

Detailed comparisons between the magnetic susceptibility and the GRAPE density record generated using the MST, and high-resolution color reflectance measured using the Oregon State University system, demonstrated complete recovery of the sedimentary sequence down to 170 mcd. The bulk densities decrease downhole with some scatter caused by varying clay content. This corresponds to the transition of sediments from nannofossil rich in the upper part to more diatom rich in the bottom of the section. Clay-rich intervals are indicated by high natural gamma radiation activity.

Hole 1022A consists of 166 m of upper Pliocene through possible uppermost lower Pliocene sediments. The Quaternary is represented only by a very thin (<1 m) veneer of sediments overlying the upper Pliocene. The total age range of the sedimentary sequence recovered from Holes 1022A and 1022B is not well constrained. Planktonic foraminifers indicate that the top of the sequence is >2.25 Ma and that the base of Hole 1022A is older than 3.3 Ma. Calcareous nannofossils indicate that the age of the base of Hole 1022A is <3.8 Ma. Because the sequence is relatively thick, each of the 4 groups examined are represented by only one or two biozones and few datums are recognized.

All of the microfossil groups in Holes 1022A and 1022B are represented by cool, relatively high-latitude assemblages. Radiolarians are entirely represented by subarctic forms. Diatoms are dominated by North Pacific subarctic assemblages. Planktonic foraminifer assemblages are dominated by subarctic to cool temperate forms, with subtropical elements absent. Both diatoms and radiolarians show evidence of strong upwelling throughout the Pliocene. The sequence at Site 1022 is the most diatomaceous of late Pliocene age of all Leg 167 sites. Radiolarians indicate a prevalence of strong coastal upwelling; whereas the diatoms reflect oscillations between strong coastal upwelling and oceanic upwelling. Benthic foraminifers throughout are typical middle bathyal, deep-sea assemblages indicative of well-oxygenated bottom waters.

Low magnetic intensities and a drilling-induced overprint precluded the establishment of a magnetic chronostratigraphy.

Volatile hydrocarbon concentrations were very high. Gases up to C6 occurred at about 100 mbsf in the sediment, probably derived from thermogenic degradation of organic material. Although methane/ethane ratios are decreasing with depth, no indication of migrated hydrocarbons was observed. Calcium carbonate values increased within the upper 100 mbsf to a maximum of about 30 wt%, and decreased again to minimum values of 5 wt% at the bottom of Hole 1022A. Organic carbon concentrations varied between 0.5 and 1.5 wt%, showing slightly increased values in lithological Unit II. According to the C to N ratios, organic matter is mainly of marine origin.

Chemical gradients in the interstitial waters reflect organic matter diagenesis, the dissolution of biogenic opal and calcium carbonate, the influence of authigenic mineral precipitation reactions, and the diffusive influence of reactions in underlying basalt. Alkalinity increases to >15 mM, whereas sulfate concentrations decrease to values below the detection limit (approximately 0.4 mM) by 37.45 mbsf. Phosphate concentrations increase to values >65 µM increase to 5.7 mM. Dissolved silicate increases to concentrations >1000 µM, and strontium increases to 182 µM. Calcium concentrations decrease to as low as 4.1 mM, then increase with increasing depth to 6.4 mM at 160.95 mbsf. Magnesium concentrations generally decrease throughout the section to 23.4 mM at 160.95 mbsf.

The 4 Adara downhole temperature measurements yield a thermal gradient of 88°C/km (Fig. 5). Using an average measured thermal conductivity of 0.950 W/(m-K) provides a heat-flow estimate of 84 mW/m2 at Site 1022.

To 167 Conclusions

To 167 Table of Contents

Publications Home

ODP Home