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doi:10.2204/iodp.proc.342.203.2017

Results

Roughly 99% of the wt% CF values range from 0.11% to 12.75% (extreme values include some samples with heavy inclusions [e.g., chert at Site U1409] and other complicating factors), with the highest values in the carbonate-rich deposits of the early Eocene at Site U1409 (6.94% ± 3.10%). Middle Eocene drift deposits at Site U1408 had lower wt% CF on average (2.24% ± 1.27%). Three lines of evidence suggest that the decline in wt% CF from the early Eocene at Site U1409 to the middle Eocene at Site U1408 does not reflect a change in preservation potential of foraminifers alone. Paleowater depth estimates put Site U1409 around 500 m deeper than Site U1408 at this time (e.g., Fig. F1), and the decline in wt% CF begins in roughly the middle of the Site U1409 record (around 50 Ma). The declining wt% CF carries on through the Site U1408 record, with some local highs in wt% CF (notably from 46–47 Ma at Site U1409). In addition, visual shipboard estimates of planktonic foraminiferal preservation quality were typically described as moderate to good in the early Eocene at Site U1409 (see the “Site U1409” chapter [Norris et al., 2014d]) and good to very good in the middle Eocene at Site U1408 (see the “Site U1408” chapter [Norris et al., 2014c]). Weight percent CF is expected to decline with a decline in preservation (Broecker and Clark, 1999), so the long-term trend across Sites U1409 and U1408 counters a pure preservation hypothesis. Similarly, increasing clay dilution with the onset of drift deposition also (alone) cannot account for the observed decline in wt% CF because the decline begins well within the early Eocene at Site U1409, before the onset of drift deposition. In addition, the decline continues, with interruptions, through the middle Eocene at Site U1408, long after the lithologically abrupt onset of deep drift deposition captured at Site U1409. In short, some portion of the long-term decline in wt% CF from (very roughly) 50 to 35 Ma likely captures a long-term decline in the mass accumulation rates of planktonic foraminifers, a hypothesis that awaits updates in age model estimates to be tested.

At Site U1408, there is general decline in wt% CF from ~43.5 to ~40 Ma, with three peaks (around 43.4–44.4, 41.1–42.2, and 39.5–40 Ma). Shipboard age models show a local peak in sedimentation rates coincident with youngest peak (39.5–40 Ma) in wt% CF. The previous two peaks (around 43.4–44.4 and 41.1–42.2 Ma) occur during an interval of generally high sedimentation rates. Whether or not higher frequency variation in wt% CF coincides with sedimentation rate changes remains to be tested when cyclostratigraphic age models are available.

Weight percent CF is generally quite low in the moderately preserved, pelagic strata of Site U1406 (0.64% ± 0.51%). Weight percent CF declines steadily from 37.4 to 36.8 Ma from roughly 1.5 to >0.5 wt% CF and remains low for the remainder of the Site U1406 record (35.7–36.7 Ma). There is no clear correlation between preservation and wt% CF from the shipboard observations of planktonic foraminiferal preservation at Site U1406. The decline from 37.4 to 36.8 Ma does coincide with a switch from good to moderate foraminiferal preservation, but the low wt% CF interval that follows (35.7–36.7 Ma) has planktonic foraminiferal preservation observations in the same range as the oldest part of the Site U1406 record (i.e., moderate to good). It is notable that Site U1406 has the deepest paleowater depth estimates of all the sites included (an estimated 3400 m at 45 Ma, compared to 2400 m at Site U1411 or 2100 m at Site U1408). Once again, however, it is unlikely that the water depth alone explains the very low wt% CFs at Site U1406. Where Sites U1411 and U1406 overlap, they show similar trends and absolute values in wt% CF and the deeper portion of the Site U1406 record having wt% CF values comparable with the shallowest samples from Site U1408.

The rapidly accumulating sediments of Site U1411 (typically between 2.27 and 5.50 cm/ky), have wt% CF values of 0.85% ± 0.61%. The late Eocene at Site U1411 has lower wt% CF (mean = 0.59%) compared to that of the early Oligocene (mean = 1.10%). Although this coincides with an overall increase in sedimentation rates (from a mean of 3.05 cm/ky in the late Eocene to a mean of 4.04 cm/ky in the early Oligocene), large variations in sedimentation rates between 2.27 and 5.49 cm/ky (shipboard estimates) in the early Oligocene do not coincide with comparable variation in wt% carbonate (Figs. F1, F3).

Although there are likely to be laboratory differences affecting wt% CF, two well-sampled intervals suggest that these effects are unlikely to influence the broad-scale patterns and inferences thereon (Fig. F2). The middle Eocene samples from Site U1408 (38.64–45.49 Ma) were washed in four primary locations: Australia National University (38.64–39.43 Ma), Yale University (39.44–40.59 and 42.22–43.58 Ma), University of Southampton (40.59–42.22 Ma), and University of California Santa Cruz (43.57–45.49 Ma). Color-coded by washing location, the Site U1408 coarse fraction data show continuous trends across washing location changes. The same is true of the completed Site U1406 washing, carried out at Heidelberg University (36.89–37.51 Ma), Stockholm University (35.66–36.89 Ma), University of Southampton (32.24–35.65 Ma), and Wasada University (32.16–35.64 Ma) (Fig. F2B) and of the Site U1411 record washed in two locations and Site U1409 washed in three locations (comparisons of Sites U1411 and U1409 are not shown).

Within Sites U1408 and U1411 there was a significant, positive correlation between sedimentation rate and wt% CF (Fig. F3) using the shipboard age models. Specifically, within-site pairwise correlation tests of linear sedimentation rate and wt% CF at these two sites have p-values less than 0.05. However, generally speaking, very little of the variance in the wt% CF was explained by sedimentation rates at these sites. Site U1408 had the strongest relationship, with a r² of just 0.112. Refined age models will provide a stronger test of the relationship between wt% CF and sedimentation rates when they become available.

The high-amplitude variability in lithology in shipboard color records and wt% carbonate (e.g., Figs. F49 and F50 in the “Expedition 342 summary” chapter [Norris et al., 2014a]) attributed to orbital forcing was also observed in the wt% CF records. In the robustly spliced interval in Magnetochron C20n at Site U1408, for instance, wt% CF records exhibit significant variability at roughly 30 and 23 ky based on shipboard age models. We suspect these two peaks correspond to obliquity and precession, although this remains to be tested with future cyclostratigraphic age model development (Fig. F4).

This wt% CF record is provided as a standalone data set that will be useful for ongoing research on Eocene paleoceanography in the North Atlantic, to alert external research groups to the availability of these samples for complementary faunal work and other geochemical applications, and to evidence progress of the consortium’s efforts to build the ESIC stable isotope stack. In addition to the benthic isotope work, many PIs in the consortium have committed to generating planktonic foraminiferal or bulk carbonate isotope records for surface and thermocline waters, and some are generating Mg/Ca and other trace metal records as well. There remain, however, thousands of samples without matching planktonic isotope records planned, and we welcome collaboration toward the second goal of producing a parallel surface ocean megastack. In addition, faunal work is planned in a limited number of intervals. Faunal splits are being made and preserved where appropriate. In the drift deposits at Sites U1408 and U1411, preservation is exceptional (glassy to near-glassy) and presents an unprecedented opportunity for long records of Eocene biotic dynamics. The unforeseen difficulty of splicing Site U1408 continuously across the middle Eocene has led to new efforts to sample and cross-correlate between Sites U1408 and U1410 in hopes of fulfilling the initial goals laid out for the ESIC during Expedition 342. Samples from Site U1410 will be added to the consortium records of wt% CF and benthic stable oxygen and carbon isotopes at a later time.

The record presented here represents the first step in an ambitious international collaboration. Already the coarse fraction record provides multiple insights in Eocene paleoceanography in the North Atlantic, including the following:

• Additional evidence for paleoceanographic change following the early/middle Eocene boundary in the steep decline wt% CF records (Fig. F1),
• Additional evidence for orbital forcing in Newfoundland Ridge drift deposits (Fig. F4), and
• Long-term variation in wt% CF records even within the drift deposits (Figs. F1, F2), in some cases possibly relating to intervals of higher average wt% carbonate noted shipboard.

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