Site 1125

Hole 1125A
Position: 42°32.9962´S, 178°9.9891´W
Start hole: 2248 hr, 2 October 1998
End hole: 2000 hr, 3 October 1998
Time on hole: 21.20 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 1376.20
Distance between rig floor and sea level (m): 11.60
Water depth (drill pipe measurement from sea level, m): 1364.60
Total depth (from rig floor, mbrf): 1579.70
Total penetration (mbsf): 203.50
Coring totals: type: APC; number: 22; cored: 203.50 m; recovered: 102.74%
Formation: lithostratigraphic Subunit IA (0–70.8 mbsf): clayey nannofossil ooze interbedded with nannofossil-bearing silty clay
lithostratigraphic Subunit IB (70.8–203.52 mbsf): nannofossil-bearing silty clay intercalated clay-bearing nannofossil ooze

Hole 1125B
Position: 42°32.9791´S, 178°9.9876´W
Start hole: 2000 hr, 3 October 1998
End hole: 1730 hr, 6 October 1998
Time on hole: 69.50 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 1377.20
Distance between rig floor and sea level (m): 11.60
Water depth (drill pipe measurement from sea level, m): 1365.60
Total depth (from rig floor, mbrf): 1929.30
Total penetration (mbsf): 552.10
Coring totals: type: APC; number: 20; cored: 188.80 m; recovered: 101.49%
type: XCB; number: 38; cored: 363.30 m; recovered: 88.09%
Formation: lithostratigraphic Subunit IA (0–74.8 mbsf): clayey nannofossil ooze interbedded with nannofossil-bearing silty clay
lithostratigraphic Subunit IB (74.8–245.2 mbsf): nannofossil-bearing silty clay intercalated clay-bearing nannofossil ooze
lithostratigraphic Subunit IIA (245.2–331.79 mbsf): clayey nannofossil chalk
lithostratigraphic Subunit IIB (331.79–552.10 mbsf): clayey nannofossil chalk

Site 1125 lies 610 km east of New Zealand's South Island at 1360 m depth on the north slope of Chatham Rise. It is in the Subtropical Convergence zone of high productivity and is swept by the East Cape Current which runs south along eastern North Island before turning east along the Rise. This current is also supplied with water (and sediment) by the Southland, which flows up the eastern South Island coast and turns to flow east in two branches, north and south of the Rise, partly through Mernoo Gap at the end of the Rise, before turning east. The area is thus richly supplied with both pelagic and hemipelagic material as well as volcanic ash from the Central Volcanic Zone.

The major targets of Site 1125 originally were to retrieve an unaltered sequence of lower Neogene and perhaps Paleogene sediments from the commencement of AAIW activity on the margin (i.e., penetrate back to the Oligocene). It appeared possible to achieve a high-quality oxygen isotopic record spanning the period of probable inception of both Antarctic glaciation and the consequent delivery of cold water into the deep circulation system, based on the appearance of a thin late Neogene section in seismic profiles. The recovered thick late Neogene sequence will provide a record of AAIW paleohydrography, changing paleoproductivity, and position of the Subtropical Convergence. It is a counterpart to DSDP Site 594 on the south side of Chatham Rise and at a similar depth (594 was at 1204 m). Site 1125 presently lies at the base of AAIW. In the North Atlantic, an intermediate water has been shown to increase both in depth range and speed during glaciations, concomitant with a decrease in North Atlantic Deep Water production in the Norwegian-Greenland Sea. Analogously, Pudsey et al. (1988) have argued that AABW production also diminished during glacials, in which case the thickness of AAIW may also have increased concomitantly. If the vigor of global deep circulation was decreased by these North Atlantic and Antarctic events, then during glacial times the Indian/Pacific upper Circumpolar Deep Water (CDW) should have become even more nutrient enriched and oxygen depleted than it is today. Material from Site 1125 (depth 1359 m) will be used for delta13C and trace-element analysis (e.g., Cd/Ca in calcite and opal) to allow ocean paleochemistry to be used to determine whether during glaciations the site lay under severely nutrient-depleted AAIW or enriched CDW.

This site yielded the thickest double–piston-cored section (Table 7) of the leg with a complete composite section for 238 mcd, extending back close to the Miocene/Pliocene boundary
(Fig. 13). Correlation was difficult, being based on very low amplitude signals in reflectance and magnetic susceptibility. The correlations are not as firmly based as at other sites and the record will probably prove difficult to tune.

The recovered sequence is divided into two units distinguished primarily on inferred carbonate content and tephra. The upper unit of Pliocene to Pleistocene age shows cyclic alternations of more and less calcareous beds and has increasing tephra upward. Below 245 m, Unit II has higher carbonate content and is more uniform in appearance. An upper subunit has occasional thin sandy glauconitic layers. The underlying subunit, from 333 mbsf to the bottom of the hole at 552 mbsf, is of clayey nannofossil chalk with numerous tephra layers. Organic carbon values are awaited, but, judging from the sulfate and methane curves, they are probably relatively high. The lower part of the section represents rapid deposition of dominantly biogenic material under high-energy conditions. Biogenic sediment declined until rapid deposition resumed at 6 Ma. Halfway through this period, terrigenous sediment increased and biogenic material declined. This may have been a result of a circulation change bringing the sediment, abundantly available since the 6 Ma change of plate motion and uplift of the Southern Alps, from the South Island coast.

Well preserved and rich calcareous nannofossil, foraminiferal, and radiolarian floras and faunas are present throughout, and rich diatom floras are present below 160 mbsf. These have yielded 47 age datums from late Pleistocene to early Miocene, thus rendering a robust age-depth curve. The flora indicates subtropical high-productivity conditions, whereas the faunas show some mixing of warm and cool subtropical species. Some cyclicity in warm/cold faunas in light/dark layers is also apparent, but no subantarctic species have been found.

Magnetic intensities are weak in the upper 200 mbsf and below that decline to the noise level of the machine. A reasonable stratigraphy can be detected in the upper 200 mbsf, with most reversals down to the base of C3n.4n at 5.23 Ma. With shore-based work, this will significantly aid the development of a tuned age model for the Pliocene–Pleistocene.

Sedimentation rates show two periods of very rapid deposition, the early Tortonian (10.1–10.6 Ma) at 190 m/m.y., and Messinian (6–5 Ma) at 15 0 m/m.y., separated by lower accumulation rates. The late Tortonian rate was ~30 m/Ma and the Pliocene to Holocene rate declined from the Messinian high to a Quaternary average of 20 m/Ma, though this may contain a 0.5-m.y. hiatus.

Physical properties at this site are very uniform with slow and slight increases in density. Magnetic susceptibility is relatively featureless. The thermal gradient is 64.9°C/km and the calculated heat flow 0.071 W/m2.

Because of severe time constraints, just one logging tool was run: the triple combination comprising gamma radiation, resistivity, and porosity/density sensors. Resistivity and density show little change over the top 500 m of the hole, but the latter jumps from 1950 to 2200 kg/m3 from around 510 mbsf to the bottom of the hole, accounting for very slow drilling encountered there. The gamma record exhibits more character which will be worth examining by spectral methods.

The upper part of the hole is marked by sulfate reduction with decline of SO4 to zero at 200 mbsf and increase of methane to 500 mbsf in the zone of methanogenesis below. Most other properties are related to these processes - alkalinity and ammonia increase in the upper 200 m and resulting Ca decrease as a result of carbonate precipitation, followed by Ca increase and carbonate dissolution in the zone of methanogenesis. Silica increases steadily downhole but sharply decreases in the bottom 40 m where particularly hard mudstone is encountered, probably caused by incipient silica cementation.

Site 1125 proved the equal of the other sites on this leg in its capacity to surprise us. Two periods of an astonishing sedimentation rate over 150 m/m.y. ensured that in 550 m the middle Miocene was not reached. The prospect for high resolution study of productivity and intermediate water masses in the SW Pacific for the last 11 m.y. is excellent, however, and comparison of this site under subtropical water can be made with Site 594 just south of the Rise under subantarctic water.

To 181 Conclusions

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