RESULTS

Changes in both sedimentation rate and composition occur over the 4-m.y. time span that we surveyed. The measured values of these various parameters are presented in Tables T1 and T2 and are summarized in individual depth plots. The linear sedimentation rate increases from 2 cm/k.y. below 370 mbsf to as great as 10.8 cm/k.y. above that depth (Fig. F1). The sand fraction comprises <2% of the total sediment below 370 mbsf; above this, its contribution increases sharply to as much as 6%, with strong cyclic variations (Fig. F2). The percentage of silt changes in similar patterns (Fig. F2).

The main components of the sand fraction are benthic and planktonic foraminifers (Fig. F3), but it also includes well-formed pyrite crystals (Fig. F4). The contribution of planktonic foraminifers increases from 40% at 420 mbsf to as high as 90% at 380 mbsf, and the contribution of benthic foraminifers decreases proportionally (Fig. F5). The amount of pyrite crystals changes in a fashion parallel to the benthic foraminifers—a continuous drop from 40% at 420 mbsf to about 10% at 370 mbsf (Fig. F4).

Accumulation rates of benthic and planktonic foraminifers (Fig. F6), whose cyclic variations are generally parallel, increase by a factor of 2-3 at 370 mbsf. Some exceptions, in which benthic foraminifers increase and planktonic foraminifers drop, occur in periods with strong fragmentation of planktonic foraminifers and point to loss of these sediment components by dissolution. In contrast, the accumulation rates of benthic foraminifers are low between 255 and 260 mbsf, whereas the planktonic foraminifer accumulation rates are rather high. These maxima occur during periods with weak carbonate dissolution, as indicated by fragmentation, and indicate good preservation of planktonic foraminifers.

Benthic foraminifer (>125 µm) accumulation rates in number per cm² per k.y. (Fig. F6) increase from <1000-3000 below 365 mbsf to >6000 at the maxima of the cycles in the uppermost Miocene. We calculated the numbers of benthic foraminifers both per gram of sediment and in number per cm² per k.y. in each fraction (Table T1). The values in all three fractions vary in parallel and show the same trend as the number per cm² per k.y. in total >125-µm fractions.

Opal skeletons of radiolarians have only been found in the interval between 285 and 325 mbsf (Fig. F7). The first maximum is simultaneous to a maximum in the accumulation rate of benthic foraminifers. Echinoids and ostracods (Fig. F8) increase above 290 mbsf. Accumulation rates of fish debris (Fig. F9) show the same trend as benthic foraminifers.

Cyclic changes of the two carbonate dissolution proxies (benthic/planktonic foraminifer ratios and fragmentation of planktonic foraminifers) (Fig. F10) vary generally in parallel, except for two intervals marked on Figure F10. From 420 to 380 mbsf, the benthic/planktonic foraminifer ratios decrease from 20%-40% to 5%-30% in the younger period. Maxima in fragmentation and mean values are higher in the 420-360 mbsf section than in the overlying unit.

The CaCO₃ concentration varies between 50 and 85 wt% (Fig. F11). The mass accumulation rate of CaCO₃ increases in three steps with time in the same way as the accumulation rate of benthic foraminifers (Fig. F12). Organic carbon percentages increase from <1 wt% below 360 mbsf to as great as 1.8 wt% in the younger section (Fig. F11). The mass accumulation rate of organic carbon increases parallel to that of CaCO₃ (Fig. F12). The C/N values vary from 19 to 7 (Table T2). Most of these atomic ratios are intermediate between unaltered algal organic matter (5-8) and fresh land plant material (25-35) (e.g., Emerson and Hedges, 1988; Meyers, 1994). Because of their setting offshore from a coastal desert, it is likely that the Site 1085 sediments contain mostly marine-derived organic matter in which the C/N values have been diagenetically elevated as a consequence of enhanced preservation (e.g., Meyers, 1997). Organic ¹³C values are mostly between -20 and -23, although values for some samples are as low as -24 (Table T3). These values are typical of marine organic matter (e.g., Meyers, 1994, 1997) and support the predominance of marine organic matter indicated by the C/N values of these samples (Table T3). Terrigenous particles have been counted in the 40- to 63-µm and >63-µm fractions (Fig. F13). The percentage in the >63-µm fraction is low (<0.5%) and percentages are up to 5% in the 40- to 63-µm fraction. Percentages decrease from 420 to 370 mbsf, but they increase again above 320 mbsf, with a maximum at 260 mbsf. The concentration of glauconite, which is believed to originate from lateral downslope transport from the shelf, has a temporal distribution like that of terrigenous particles (Fig. F14). The highest values of this mineral are found at 260 mbsf and are coincident with maxima in terrigenous matter.