The chronostratigraphy of Site 1144 is primarily derived from calcareous nannofossil and planktonic foraminiferal zones and events. Because of the extremely high sedimentation rates at this location, only eight biostratigraphic zones and markers were identified (see "Biostratigraphy"). Unfortunately, the magnetopolarity stratigraphy was also limited because of high sedimentation rates and magnetic overprint; only the Laschamp Event (0.04 Ma) is tentatively identified at 23.5 to 25.5 mcd. Although not highly detailed, the biostratigraphic control points proved adequate for calculating sedimentation rates (Table T6). Because of intensive bioturbation, sediment redeposition is extremely difficult to identify at this site.
Figure F17 (also given as Synergy Software KaleidaGraph plots and Microsoft Excel data files [see the "Supplementary Materials" contents list]) shows the linear sedimentation rate and mass accumulation rate curves based on calculations described in "Sedimentation and Accumulation Rates" in the "Explanatory Notes" chapter. The sedimentation rates varied from a maximum of ~1000 m/m.y. over the interval ~100-242 mcd to a minimum of 324 m/m.y. for the interval 242-396 mcd (Table T9). When converted to mass accumulation rates (g/cm2/k.y.) and partitioned into carbonate and noncarbonate components, extremely high accumulation rates are observed over the past 0.26 m.y. (Fig. F17). Accumulation rates in the mid-Pleistocene (1.05-0.78 Ma) are high (~60 g/cm2/k.y.) with noncarbonate components dominating. From 0.78 to 0.26 Ma, the mass accumulation rate was relatively constant (~37 g/cm2/k.y.). Then it increased dramatically to a maximum of >100 g/cm2/k.y. (especially the noncarbonate). The rate remained high but decreased gradually toward the present. These extremely high noncarbonate accumulation rates likely reflect the onset of active drift-type deposition at Site 1144. This onset may be related to changes on the shelf that direct more fine-grained sediment to the site or to a change in the circulation that transports the material.