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doi:10.2204/iodp.proc.308.201.2008

Results

The original planktonic isotope results are shown in Table T1 and plotted in Figure F1. The results show that, in Hole U1322B, samples vary between –3‰ and 0.5‰ for δ¹⁸O and between –0.2‰ and 1.8‰ for δ¹³C. The lightest δ¹⁸O value of about –3.12‰ occurs at 7.85 mbsf, implying the influence of freshwater discharge from the Mississippi River during the early Holocene warm or freshwater plume from the melting of the Laurentide Ice Sheet (Kennett and Shackleton, 1976). At 90.34 mbsf, the δ¹⁸O value is also very low, –3.0‰, likely corresponding to a warm period of marine isotopic Stage (MIS) 3. In contrast, the heaviest value of 0.53‰ is found at 86.84 mbsf, probably indicating the Last Glacial Maximum.

Because the sediment sequence recovered in Hole U1322B contains many slumped intervals indicative of mass flow (Expedition 308 Scientists, 2005), 35 samples from these intervals are not considered as being autochthonous and therefore their isotopic results cannot be used. Subsequently, only 62 samples are left to represent the in situ sediment recovered in Hole U1322B (Fig. F1).

However, some of these 62 samples show rapid increases or decreases in δ¹⁸O values, especially those swift changes to heavier values from immediately below or above slumped sediment layers. Sudden swings in δ¹⁸O values from tropical areas often characterize glacial–interglacial transitions or short-term climate events, but many such swings on the δ¹⁸O curve from Hole U1322B appear to be abnormal. Similarly “abnormal” δ¹⁸O readings, however, are widely reported from the Gulf of Mexico and regions affected by freshwater bursts near big rivers and diagenetic overprinting due to slumps (e.g., Williams and Kohl, 1986; Johnsen et al., 2001). Therefore, 12 such isotopic values from Hole U1322B likely affected by diagenesis (sudden heavy δ¹⁸O values) and by freshwater (sudden very light δ¹⁸O values) should be disregarded (Fig. F2).

Apart from the minimum and maximum oxygen isotopic values that respectively denote warm and cold climate stages, a proper designation of MIS for the Hole U1322B record needs comparisons with the global standard as well as other local records. Overall, Hole U1322B δ¹⁸O and δ¹³C curves can be compared to that of ODP Site 619 (Pigmy Basin) and Core MD03-2642 (Brazos-Trinity Basin #4) on the Louisiana–Texas continental slope, west of Site U1322 (Fig. F1). A closer examination indicates that the first two are more comparable, but δ¹⁸O values from Core MD03-2642 are much lighter for the MIS 1–4 interval. Mallarino et al. (2006) attribute the light δ¹⁸O values in Core MD03-2642 to the use of <150 µm specimens of white and pink G. ruber because small and pink-pigmented specimens often generate lighter isotopic readings.

The modified δ¹⁸O curve for Hole U1322B using the remaining 50 samples (Fig. F2) shows a better match with the MIS 1–4 interval of the low-latitude isotopic stack (Bassinot et al., 1994). δ¹⁸O variations follow a trend similar to other curves in indicating a time interval from the last glacial–interglacial cycle since ~70 ka. Biostratigraphic results from Hole U1322B also reveal changes in planktonic foraminifer and nannoplankton assemblages with a pattern of “warm → cold → less warm → cold” downhole (Expedition 308 Scientists, 2005). Therefore, these comparable isotopic and biotic results indicate that the sediment sequence recovered in Hole U1322B was largely deposited during MIS 1 through MIS 4. Respectively, the MIS 1/2, 2/3, and 3/4 contacts can be drawn close to 23, 89, and 200 mbsf.

Similar to other records from the northern Gulf of Mexico, the Hole U1322B δ¹⁸O record shows the typical isotopic variation trend found in the tropical–subtropical deep sea (e.g., Bassinot et al., 1994) only after omitting some unusual and unreliable values (Fig. F2). These unusual and unreliable δ¹⁸O values are interpreted as mainly affected by rapid freshwater discharges from the Mississippi River during ice melting and diagenetic overprinting across boundaries between slumped and in situ sediment intervals. The results from this coarse-resolution study help to establish a preliminary oxygen isotopic stratigraphy for Site U1322. A more detailed correlation with high-resolution tropical and high-latitude δ¹⁸O records requires closer sampling of the sediment section in future studies.

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