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doi:10.2204/iodp.proc.342.102.2014

Age models and mass accumulation rates

Age models

The age models developed for each site are based on shipboard micropaleontology and paleomagnetic measurements. For most sites, the age model is based on Hole A, which was the hole that was examined in the most detail by the micropaleontology team. The age-depth model was constructed from tabulations of biostratigraphic and paleomagnetic datums. Datums were placed at midpoints between bounding samples for biostratigraphy and between inflection points in the declination or inclination curves for magnetochron boundaries. The bounding samples and inflection points represent datum uncertainty in age-depth plots. Age determinations are based on GTS2012. See Figure F5 and Tables T1, T2, and T3 for a more detailed description of this timescale and biostratigraphic datums, including notes on additional biostratigraphic datums utilized during Expedition 342; see “Paleomagnetism” for a detailed description of the GPTS utilized during Expedition 342.

Linear sedimentation rates

Shipboard micropaleontology and magnetostratigraphy were used to construct a linear sedimentation rate (LSR) curve for each site. For Sites U1403–U1410, depths are based on Hole A. For Site U1411, depths are based on Hole B, which was the principal first hole. Construction of the LSR curve was an iterative process involving refinements of magnetochron identification coupled with review of the reliability of specific biostratigraphic datums. The final LSR curve is drawn through the finalized paleomagnetic datums and the most robust biostratigraphic datums, taking care to avoid age inversions. Because of known calibration uncertainties (Gradstein et al., 2012) and the potential for significant diachroneity (e.g., Sanfilippo and Blome, 2001), we chose to construct LSR curves based on a selection of tie points rather than incorporate all biostratigraphic datums in the age model. This strategy minimizes the potential for spurious variations in LSR and mass accumulation rate (MAR) over short timescales that can result from poorly calibrated or diachronous biostratigraphic events.

Datum reliability was evaluated on the basis of

1. Uncertainty relating to the shipboard sampling interval and intervals of poor microfossil recovery,

2. Reliability of the calibration of the datum to GTS2012, and

3. Relative reliability of datums when datums were in contradiction.

Selected datum ties were chosen as the tie points for LSR line segments.

Datum tie points are denoted by a tie point number that increases with depth (e.g., D01 to D12 in Table T18 in the “Site U1403” chapter [Norris et al., 2014]). The slope of line segments between datum ties is the LSR used in the MAR calculations. Using this approach, the LSR creates a square wave pattern that is imparted to the MAR record; this artifact must be considered when interpreting MAR calculations.

Clusters of biostratigraphic events of varying ages at or close to specific depth horizons were used to infer hiatuses. Hiatuses were also inferred where shipboard magnetostratigraphy could not be continuously correlated to the GPTS or became inconsistent with well-calibrated biostratigraphic datums. Where possible, these hiatuses were verified with reference to physical property and lithostratigraphic data.

Mass accumulation rates

MARs were calculated for the Expedition 342 drill sites using (1) LSR derived from shipboard age-depth models and (2) dry bulk density calculated from shipboard moisture and density (MAD) analyses. MARs of carbonate (CAR) and noncarbonate (nCAR) fractions were then calculated by multiplying the MAR by carbonate weight percent, which was calculated from shipboard coulometry measurements of inorganic carbon (IC) weight percent, where

This calculation assumes that all of the inorganic carbon is present as calcite and aragonite.

Dry bulk density

Shipboard MAD measurements provided wet mass, dry mass, and dry volume values for discrete samples at a resolution of one sample per section (see “Physical properties”); dry bulk density values were calculated from these data. All of these data are tabulated in the “Age-depth models and mass accumulation rates” section of each site chapter. The shipboard age models were applied to the dry bulk density records.

Time step for mass accumulation rate calculations

The time step for the MAR calculations was selected on the basis of the sampling resolution of the shipboard MAD samples. Considerations taken into account for the time step of MAR calculations are (1) consistency in the time step between sites and (2) the geological period of interest that is best represented in each site. A 200 k.y. time step was chosen and the dry bulk density (DBD) and carbonate data were resampled at this new sampling interval using the Analyseries software. Average values between age midpoints were calculated using linear interpolation. MAR, CAR, and nCAR values (g/cm²/k.y.) for each 200 k.y. time step are therefore

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