RESULTS

General Characteristics of the Planktonic Foraminifer Assemblage

At least 5%–10% of planktonic foraminifer tests are seriously affected by dissolution and became fragmented. More fragments are found in interglacial intervals (maximum = 60%–80%) than in glacial intervals (mostly 10%–15%). There are exceptions, however, with up to 30%–40% fragmented tests from some glacial intervals (Fig. F2).

Except where dissolution prevails, planktonic foraminifers are abundant and well preserved in most samples studied. The absolute abundance varies from <30 to >400 specimens/g of dried sample. (Fig. F2). More species and specimens are recorded in interglacial intervals than in glacial intervals, but the absolute abundance may increase to 300–500 specimens/g at some levels of glacial marine isotope Stages (MIS) 16 and 22, signaling sudden blooms of certain cool-water forms.

Representing the subtropical province (Bé, 1977), the planktonic foraminifer assemblage is dominated by Globigerinoides (Globigerinoides ruber and Globigerinoides sacculifer) and Globorotalia (warm-water Globorotalia menardii and cool-water Globorotalia inflata) and Neogloboquadrina (mainly Neogloboquadrina dutertrei). There is a major shift in the abundance of warm and cool groupings at ~420 mcd, with warm species averaging >50% below this level and cool-water individuals increasing their average abundance to 35% or more above this level. Fluctuations are evident in all groupings, and they are mainly related to the alternation of glacial–interglacial cycles (Figs. F3, F4, F5).

Geochronologic Framework and Useful Species Datum Levels

The shipboard geochronologic framework for Site 1144 was mainly based on nannofossil and planktonic foraminifer biostratigraphy and geomagnetostratigraphy (Wang, Prell, Blum, et al., 2000). The age model has since been refined by postcruise isotopic studies (Bühring et al., this volume). The isotopic results indicate that the interval between 300 and 500 mcd fall in MIS 14–29, representing sedimentation in the past 0.5–1.0 m.y. (Table T1).

Several biostratigraphically useful foraminifer events were also observed (Table T1). In descending order, they are as follows:

The first occurrence (FO) of pink G. ruber at 313 mcd is in the lower part of MIS 14. This datum bears an age of ~0.55 Ma, about 0.13 m.y. older than its consistent occurrence level dated previously at 0.42 Ma (Li, 1997; Wang, Prell, Blum, et al., 2000).
The last occurrence (LO) of Stilostomella at ~356 mcd is in the lower part of MIS 17 with an age of ~0.69 Ma, or ~0.07 m.y. older than the record of 0.62 Ma from other tropical Indo-Pacific regions (Schönfeld, 1996) but ~0.06 m.y. younger than the 0.75 Ma dated at Site 1143 in the southern South China Sea (Wang, Prell, Blum, et al., 2001).
At 500.50 mcd, the coiling direction in Pulleniatina obliquiloculata changed from sinistral to dextral. This probably marks the last such coiling change in this taxon and occurred ~1.6 m.y. ago (Kennett and Srinivasan, 1983; G.X. Qin, 2002, pers. comm.). This finding may have two implications: (1) the bottom part of the core at Site 1144 (519 mcd) could have an age of ~1.7 Ma or older (rather than 0.9 Ma as suggested in the Leg 184 Initial Reports volume) and (2) a hiatus lasting up to 0.6 m.y. (between 1.0 and 1.6 Ma) could have occurred near the 500-mcd level. Immediately above this level, at 499.19 mcd, the LO of the small Gephyrocapsa acme has been dated as representing 1.01 Ma (Wang, Prell, Blum, et al., 2000). An age >1.0 Ma for the bottom part of Site 1144 is also supported by Neogloboquadrina humerosa, a late Miocene to early Pleistocene species (8.5–1.3 Ma) found mainly in intervals below 479.86 mcd. In the following, however, we refer to the intervals below 500 mcd as "intervals older than MIS 29" pending further studies of its exact age.

The isotope results by Bühring et al. (this volume) indicate that the mid-Pleistocene revolution (MPR) (0.9 Ma) lies at ~418 mcd, close to the MIS 22/23 boundary (Fig. F2). Moreover, the Brunhes/Matuyama boundary, which was not well defined in the Leg 184 Initial Reports volume, should be tuned to 386 mcd to coincide with the early part of MIS 19. Abundant microtectites were also observed at a similar level (~386 mcd), representing the meteorite impact event widely recorded in the Indo-Pacific region (Glass, 1967; Zhao et al., 1999). These two events appear to have concurred at ~0.78 Ma (Fig. F2).

Abundance of Planktonic Foraminifer Species

Globigerina bulloides d'Orbigny
Remarks: Globigerina bulloides is extremely rare (0%–3%) in the bottom 40 and top 20 m of the studied section. Its abundance increases to 5% or more in glacial cycles, especially MIS 16, 18, 20, and 22, between 320 and 420 mcd. Interestingly, a similar percentage is also recorded from the upper part of MIS 19 and 17, where other cool-water forms such as Globorotalia inflata and Neogloboquadrina pachyderma also register relatively high abundances (Fig. F4).

Globigerina falconensis Blow
Remarks: Similarly rare as G. bulloides, G. falconensis appears to be less influenced by glacial–interglacial alternations. After MIS 19, however, its abundance maxima (2.5%–4%) always occurs during interglacials (Fig. F4).

Globigerinella aequilateralis (Brady)
Remarks: Rare throughout the section, G. aequilateralis constitutes only 2% or less in the lower part of the section, increasing to 3% or more after MIS 19, and peaking at two levels within MIS 18 and 16 with over 4% abundance. It decreases again upsection in MIS 15 and 14 (Fig. F5).

Globigerinoides ruber (d'Orbigny)
Remarks: As a typical subtropical species, G. ruber is frequent to abundant, averaging 18%–20%. A maximum abundance of >40% is found at 510 mcd. Generally, its abundance curve shows a parallel trend with isotopic fluctuations, although details vary and the trend may reverse to high G. ruber abundance in some parts of glacial intervals, such as in upper MIS 26 and lower MIS 20 (Fig. F3).

Globigerinoides sacculifer (Brady)
Remarks: This tropical–subtropical species is more frequent between 455 and 490 mcd (average = 20%) than in other intervals (average = 15%). A maximum abundance of ~50% occurs at 457 mcd. Low G. sacculifer abundance associated with glacial intervals, however, is evident only after MIS 23 (above 420 mcd), and its abundance increased in the upper parts of MIS 18, 16, and 14 (Fig. F3).

Globigerinoides conglobatus (Brady)
Remarks: Unlike G. ruber and G. sacculifer, G. conglobatus is rare, often <5%. Its abundance increases to >7% only in MIS 21 and older interglacial stages (Fig. F3).

Globorotalia menardii Parker, Jones, and Brady
Remarks: This tropical–subtropical species shows distinct correlation with interglacial cycles above 420 mcd, but such a positive correlation is vague from below this level, although several abundance peaks all fall within interglacial intervals. It is extremely rare or even absent from upper MIS 22, mid-20, upper 18, upper 17, 16, and lower 14 (Fig. F3).

Globorotalia truncatulinoides (d'Orbigny)
Remarks: This deepwater thermocline dweller is rare, 4% or less, in most samples. Its abundance may increase to 4% or more in interglacial stages but decreases even to zero in interglacial intervals. A close association with glacial–interglacial fluctuations is more eminent after MIS 22 (above 410 mcd) (Fig. F5).

Globorotalia inflata d'Orbigny
Remarks: Rare to frequent G. inflata, mainly 10% or less, occurs below 420 mcd and in interglacial to glacial transitions above this level. Its overall abundance increases significantly above this level, reaching 25%-30% at upper MIS 22, upper 20, upper 19, upper 17, lower 16, upper 16, and lower 15 (Fig. F4).

Other Globorotalia
Remarks: These include Globorotalia tumida (Brady) and Globorotalia hirsuta (d'Orbigny). They are rare, each rarely exceeding 3%, and occur mainly in interglacial intervals (Fig. F5). Comparatively, G. hirsuta has a more discontinuous range.

Neogloboquadrina dutertrei (d'Orbigny)
Remarks: This species shows a steady increase upsection in its average abundance, from ~15% below 420 mcd to 20%–25% between 420 and 320 mcd, to >30% between 320 and 300 mcd. Its low abundance often occurs at glacial–interglacial or interglacial–glacial transitions, while its abundance maxima are mainly confined to glacial cycles, especially MIS 14, 16, 20, 22, and 26 (Fig. F4).

Neogloboquadrina humerosa (Takayanagi and Saito)
Remarks: Rare typical N. humerosa mainly occurs below 440 mcd, becoming more frequent below 500 mcd.

Neogloboquadrina pachyderma (Ehrenberg)
Remarks: This cold-water species is rare, often <5%, except in lower MIS 29, upper 22, upper 19, and upper 18, where its abundance may reach 10% or more (Fig. F4).

Orbulina universa d'Orbigny
Remarks: Except at several interglacial levels where its abundance exceeds 10%, O. universa attains only 2%–5% of the total planktonic foraminifer assemblage in most samples. Above 420 mcd, it remains frequent in MIS 21, 19, upper 18, 16, and lower 15 (Fig. F5).

Pulleniatina obliquiloculata (Parker and Jones)
Remarks: The abundance of this subtropical species varies from <5% to >15%, with two abundance pulses of ~40% at 415 and 422 mcd. Its abundance fluctuations are often associated with glacial–interglacial cycles, but this coherence is less obvious from below 420 mcd, prior to MIS 20 (Fig. F5).

Sphaeroidinella dehiscens (Parker and Jones)
Remarks: This tropical–subtropical dissolution-resistant species is rare (0%–3%) above 420 mcd, increasing occasionally to 5% or more downhole. Its abundance peaks occur in both glacials (upper MIS 24 and upper 26) and interglacials (mid-MIS 25, mid-27, and mid-29) (Fig. F5).