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doi:10.2204/iodp.proc.313.203.2019

Introduction

X-ray fluorescence (XRF) is a powerful tool for the analysis of marine sediments and has been commonly utilized during the Integrated Ocean Drilling Program (IODP). For example, XRF can be used to distinguish lithologic and mineralogic variation, to correlate across sites, to provide high-resolution geochemical records, and to add insight into syn- and postdepositional processes (e.g., Jansen et al., 1998; Kuhlmann et al., 2004; Lyle et al., 2012; Türke et al., 2014; Rothwell et al., 2006; Penkrot et al., 2017).

During IODP Expedition 313, siliciclastic sediments were recovered from Holes M0027A, M0028A, and M0029A. Each hole was drilled, cored, and logged through a series of Miocene clinoforms offshore New Jersey (Fig. F1A). These clinoforms are marked by recognized seismic reflectors, many of which were dated and correlated using microfossils in sediments recovered during Expedition 313. The siliciclastic sequences were divided into eight lithologic units that share similar sedimentological characteristics across holes (see the “Expedition 313 summary” chapter [Expedition 313 Scientists, 2010a]; Miller et al., 2013). The uppermost Miocene clinoform sequence analyzed in this report (Fig. F1B) lies between seismic reflectors m5.2 and m4.1 and in lithostratigraphic Unit II (Expedition 313 Scientists, 2010a) and has been dated as the mid-Miocene (Browning et al., 2013). This interval, therefore, lies in a period of significant climate change during global cooling following the middle Miocene climate optimum (Zachos et al., 2008).

Petrophysical data from this interval display interesting characteristics, including electrical conductivity changes that correspond to changes in pore water salinity (see the “Expedition 313 summary” chapter [Expedition 313 Scientists, 2010a]; Lofi et al., 2013), regions of high and variable magnetic susceptibility (Expedition 313 Scientists, 2010a) that correlate with rock magnetic changes (Nilsson et al., 2013), acoustic image surfaces that are not matched by sedimentological changes (Expedition 313 Scientists, 2010a), and intervals of high sonic velocity that correspond to cemented intervals (Expedition 313 Scientists, 2010a; Miller et al., 2013). In the holes that are more proximal to land (Holes M0027A and M0028A), this interval includes extended clay sequences where core recovery was >100%, suggestive of clay expansion, which has distinctive petrophysical characteristics (Expedition 313 Scientists, 2010a; Inwood et al., 2013; Inwood, 2018). However, correlation between the two more landward holes is not without ambiguity, nor is the stratal significance and precise locations within the core of seismic reflector m4.1 fully determined (Expedition 313 Scientists, 2010a; Miller et al., 2013). In Hole M0027A, diatoms die out at the base of the clay sequence (Barron et al., 2013), and terrestrial/marine ratios and foraminiferal observations indicate considerable water-depth variations (Katz et al., 2013; McCarthy et al., 2013; Kotthof et al., 2014). Furthermore, despite the interval further offshore in Hole M0029A representing an expanded succession, the distinctive petrophysical characteristics observed at the more proximal sites are not apparent, suggesting that these sediments were either eroded or not originally deposited. Therefore, the acquisition of geochemical data (XRF measurements on the split-core surface and on core samples) represents an opportunity to elucidate the depositional and postdepositional geochemical changes and produce a more complete story of the evolution of this interval.

The major objective of this report is to describe and evaluate the new XRF data obtained primarily from the interval between seismic reflectors m4.5 and m4.1, including 30 m of very high resolution measurements in Hole M0027A in the interval characterized by distinctive petrophysical changes. Major geochemical variations and trends are briefly discussed in relation to changes in lithology, seismic surfaces, pore water, and inferred depositional environments.

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