Previous   |    Next

doi:10.2204/iodp.proc.308.202.2007

Results

The original census data of planktonic and benthic foraminifers are shown in Tables T1 and T2. In general, both planktonic and benthic foraminifers are relatively rare in most samples. As shown in Figure F2, only a few samples from between 150 and 200 meters below seafloor (mbsf) have more than 0.5% of the >63 µm coarse fraction, indicating a mud-dominated lithology throughout. Counts of planktonic foraminifer individuals are usually fewer than 25 per 50 g dry sediment and only occasionally reach 100 or more. As at other Gulf of Mexico settings, benthic foraminifers are relatively more abundant, usually 25–200 individuals per 50 g dry sediment. Samples with 400 or more benthic foraminifer specimens are from two intervals: 0–11 and 85–183 mbsf (Fig. F2).

A number of slumped intervals found in Hole U1322B (Fig. F2) represent mass flow deposits (Expedition 308 Scientists, 2005). About 50 samples from these intervals are not considered in situ, and therefore these results should be excluded from discussion about environmental changes. However, a common feature of these samples is that both planktonic and benthic foraminifers are much less abundant than in nonturbidite layers, probably as a result of rapid redeposition of shallower water sediments. These intervals appear to have been associated more closely with sea level lowstands at or close to MIS 2 and MIS 4 (Li et al., this volume).

Planktonic foraminifer zonation

Figure F3 illustrates the relative abundance of some common planktonic foraminifer species. It is obvious that the assemblage is dominated by Globigerinoides ruber and Globorotalia inflata, with frequent Globigerinoides sacculifer, Globorotalia crassaformis, and Globigerina bulloides and its allied Globigerina falconensis, forming a typical subtropical to warm temperate association. Species restricted to the tropics, such as Globorotalia menardii and Pulleniatina obliquiloculata are rare, as are cool-water species that live at high latitudes, such as Neogloboquadrina pachyderma and Globigerina quinqueloba.

The typical warm-water species G. menardii is found in samples from Core 308-U1322B-1H (0–4 mbsf) only, whereas P. obliquiloculata occurs at 0.8, 136.6, 150, 158, and 167 mbsf. Moderately abundant G. sacculifer (~10% or more) occurs in three intervals: 0–2, 15–21, and 70–92 mbsf (Fig. F3). Over 50% abundance of G. ruber occurs at 0–20 mbsf and at 90 and 196 mbsf. Globorotalia truncatulinoides, a deep-dwelling species, has >10% abundance at 37 and 132–141 mbsf. In contrast, G. inflata and G. bulloides groups, both representing the temperate assemblage, show high abundances of ~20% in the intervals 20–70, 13–155, and 198–230 mbsf (Fig. F3).

These distribution patterns of planktonic foraminifers suggest that several Gulf of Mexico faunal (sub)zones, as modified by Kennett and Huddlestun (1972), can be recognized.

Zone Z (0–4 mbsf) is defined by the presence of G. menardii, G. sacculifer, G. crassaformis, P. obliquiloculata, and other warm species. The cool-water species G. inflata is absent from this zone.

Subzone Y1 (4–25 mbsf) contains no G. menardii but abundant G. sacculifer and low G. inflata. According to Kennett and Huddlestun (1972), Subzone Y1 represents an interval of meltwater deposition at the MIS 1/2 transition ~10.5–16 k.y. ago. Subzone Y2 (25–69 mbsf) has abundant G. inflata decreasing upward and represents deposition during MIS 2. Subzone Y3 (69–141 mbsf) is characterized by abundant G. sacculifer and G. crassaformis, with an abundance peak of G. truncatulinoides at the base. According to Kennett and Huddlestun (1972), Subzone Y3 corresponds to the later part of MIS 3, with an updated age of ~25–40 ka (Li et al., this volume). Below 141 mbsf is an assemblage with strongly variable abundances of G. ruber, G. crassaformis, and G. inflata probably representing Subzones Y4 and Y5, but their division is hampered by frequent slumping (Fig. F3). However, several specimens of the warm-water species P. obliquiloculata occurring in the interval 150–167 mbsf appear to represent in situ deposition at warm spells in the early part of MIS 3.

Benthic foraminifer assemblages

Benthic foraminifers are dominated by infaunal species living below the sediment/water interface. They often possess a small, elongate test, such as Bolivina (including Brizalina), Bulimina, Uvigerina, and Virgulina. Some are larger and ovular in shape, including Globobulimina and Chilostomella. Typical epifaunal taxa, such as Cibicides, Cibicidoides, Gyroidina, Gyroidinoides, and Oridorsalis, are rare except in the uppermost 5 m of the core (20% or more) and a few >10% peaks close to 20, 130, and 155 mbsf (Fig. F4). Miliolids (mainly Pyrgo, Triloculina, and Quinqueloculina) and Ammonia beccarii occur sporadically throughout the sediment section, with high frequencies in slumped intervals, especially at 100–121 mbsf (miliolids) and close to 201 mbsf (A. beccarii), suggesting they are probably reworked (Table T2). On the basis of dominant taxa, four infaunal assemblages and one epifaunal assemblage can be recognized.

The four infaunal assemblages are the Bolivina assemblage, Bulimina assemblage, Uvigerina assemblage, and Globobulimina assemblage. The Bolivina assemblage features abundant B. alata and B. striatala and occurs mainly in the intervals 35–85 and 150–162 mbsf. The Bulimina assemblage contains abundant B. aculeata, B. mexicana, and B. exilis and occurs mainly in the intervals 4–90 and 141–183 mbsf. The Uvigerina assemblage is characterized by frequent U. peregrina and U. globulosa in three intervals: 0–1, 10–15, and 127–132 mbsf. The Globobulimina assemblage, with 20% or more G. affinis, is found at 7–26, 68–70, and 165–172 mbsf, with more than 60% abundance in the upper two intervals. In addition, the concentration of Chilostomella ovoidea (5%–15%) between 80 and 171 mbsf is also significant. The epifaunal assemblage is characterized by 10% or more Cibicides, Gyroidina, Oridorsalis, and allied forms, found mainly at 0–8, ~20, ~130, and ~155 mbsf (Fig. F4).

Modern benthic foraminifer assemblages from the northern Gulf of Mexico show distinct depth-dependent characteristics (Poag, 1981). Offshore from the Mississippi River mouth, Uvigerina facies occurs at ~180 m, Brizalina (=Bolivina) facies at ~180–900 m, Bulimina facies at ~900–2200 m, and Nuttallides facies and Cibicides facies in waters below 2200 m (Fig. F1). Therefore, changes in the abundance and distribution of benthic foraminifer assemblages in Hole U1322B could have been affected by river discharge and slumping intensities, as well as by water depth fluctuations due to sea level changes in late Pleistocene glacial–interglacial cycles.

Top of page   |    Previous   |    Next