Palmer Deep Sites (1098 and 1099)

Sites 1098 and 1099 (Figs. 2, 18) lie in the inner-shelf basins of Palmer Deep, south of Anvers Island, where an expanded, mainly biogenic sediment record has accumulated since the retreat of the ice sheet. The prime site, 1098, was in the small, narrow Basin I, in 1012 m water depth. Site 1099 was in Basin III, a larger basin at 1425 m water depth. Seismic profiles had shown that sediments in Basin I were thinner than in Basin III (Fig. 19), but short piston cores showed that the sediment record in Basin I was less prone to deposition of locally derived turbidites. Drilling would provide an ultrahigh-resolution paleoclimatic record of the Holocene that would show short-term (200 yr) and longer-term (2500 yr) cyclic variation most probably related to global climate variation. It would then be compared with similar records from drilling in the Cariaco Basin, Santa Barbara Basin, and Saanich Inlet and with the record from ice cores.

Three APC holes extended to basement at 47 mbsf at Site 1098, and at Site 1099 a single section was recovered to 108 m (Fig. 20). At Site 1098 we recovered diatom oozes to diatom bearing mud. Deposition was by pelagic settling of diatom blooms and by gravity flows from the steep margins of the basin. Three prominent silt or sand turbidites occur between 23 and 40 mbsf. The uppermost is nearly 4 m thick, graded from diatom-bearing sandy muds to ooze, and shows a magnetic-susceptibility high induced by the terrigenous component. Slumped and inclined diamict and thin-graded fine sand-silt beds were seen at the base of the holes, but most of the sequence at Site 1098 is horizontally bedded and finely laminated.

Laminae are diatom oozes of different species composition. Microfossil preservation is excellent, and this site will provide an unparalleled record of Antarctic Holocene paleoproductivity. Variation in magnetic susceptibility may show 200- and 2500-yr cyclicity in broad intervals of high susceptibility from 0 to 8 mbsf and 30 to 38 mbsf. Cores provide an extremely high resolution record of variation in geomagnetic field direction. GRAPE densities show the turbidite sequences between 24 and 30 mbsf, superimposed on a general trend of increasing density with depth. The approach to basement is marked by a greater increase in density coincident with the lithologic change to terrigenous silt and sand at around 42 mbsf. Natural gamma levels may show some cyclicity on the order of 10 m while generally increasing downhole. Sampling for interstitial water at unusually high resolution (one per section) resulted in puzzling profiles of interstitial water chemistry at Site 1098. The sulfate reduction zone lies between 20 and 30 mbsf. Organic matter decay has surprisingly little effect on interstitial water chemistry above the sulfate reduction zone. Despite relatively high organic carbon content (1.0-1.2 wt%) and extremely rapid sedimentation, alkalinity, ammonium, and phosphate, direct by-products of organic matter decay, increase only slightly with depth in the upper 20 m. We speculate that the tightly encased Chaetoceros spores, the majority of diatoms in Palmer Deep, render organic material unavailable for bacterial consumption until dissolution breaches the external casing.

Site 1099 recovered a diatom ooze section like that of Site 1098 but greatly expanded by biogenic turbidites (Fig. 20). The lowest 20 m contained black, organic-rich, laminated diatom ooze. The major diatom species at Sites 1098 and 1099 reveal a downward trend of decreasing open-ocean influence and increasing sea-ice and basin restriction. Other microfossils show similar trends: radiolarians are common and well preserved in the upper 28 mbsf of Site 1098 with relatively diverse assemblages, but diversity and abundance gradually decrease to zero at the bottom of the hole. At Site 1099 the pattern of decreasing abundance and diversity downcore is similar, but it occurs over 108 m (the record was expanded because of increased terrigenous supply). Benthic foraminifers are well preserved and abundant in the upper 87 m of Hole 1099A. They appear similar to previously reported benthic foraminiferal fauna of the Bellingshausen Sea, fluctuating between open ocean (Circumpolar Deep Water [CDW]) and shelf-water conditions and tied to 200-yr productivity cycles. However, beneath 87 mbsf in Hole 1099B, the depth of the highest black interval, the microfossil assemblages suggest more restricted oceanographic conditions (decreased CDW influence and colder shelf waters). Samples at the bottom of the hole contain large amounts of pyrite and a few foraminifers.

The sulfate reduction zone at Site 1099 lies between the seafloor and 10 mbsf, shallower than at Site 1098, and is accompanied by a steady downward increase in alkalinity and ammonium. The unusual interstitial water chemistry profiles of Site 1098 are not found at Site 1099, but organic carbon contents are comparable. Overall, at Palmer Deep sites, organic carbon content was 1.0-1.5 wt%, and the effects of organic matter decay were an order of magnitude greater than at the continental rise sites (1095, 1096, and 1101).

MST data for the three holes at Site 1098 allow good correlation between holes to the full depth and match previous measurements on shallow cores in their upper part. The two broad intervals of high magnetic susceptibility at 0-8 and 30-38 mbsf are also marked in gamma-ray attenuation density. Deep-tow boomer records around Site 1098 show an upper stratified seismic unit and a lower, more transparent, unit. The base of the stratified unit corresponds at Site 1098 to the change from mainly bioturbated to mainly laminated sediments. At Site 1099, using a synthetic seismogram derived from MST data, eight distinct reflectors in the deep-tow boomer section correlate exactly with distinct turbidite horizons. A widespread, strong mid-basin reflector beneath them coincides with a sand and pebble layer at 33 mbsf.

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