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doi:10.2204/iodp.proc.307.206.2009

Introduction

This report provides the results of carbon isotopic analyses performed on total dissolved inorganic carbon (DIC) on 111 samples of interstitial water recovered from Integrated Ocean Drilling Program (IODP) Sites U1316–U1318 during Expedition 307 to the Porcupine Seabight. The seabight, located southwest of Ireland, is known for extensive development of deepwater coral mounds up to 160 m high that occur in water depths of 600–1000 m (Freiwald and Roberts, 2005). A major objective of IODP Expedition 307 was to investigate two competing hypotheses regarding the initiation and development of deepwater coral mounds: that mound development is (1) controlled by changes in ocean circulation and climate (Mullins et al., 1981; Dorschel et al., 2005) and (2) related to the presence of light hydrocarbon seeps (Hovland et al., 1994, 1998). In this regard, the expedition focused on the origin and development of Challenger Mound (52°23′N, 11°43′W), which lies at a water depth of ~800 m in the Belgica mound province on the southwest-facing slope of Porcupine Seabight (see the “Expedition 307 summary” chapter).

Challenger Mound was penetrated five times during Expedition 307 (Holes U1317A–U1317E). Examinations of the resulting cores indicate that the mound developed atop a major unconformity between the middle Miocene and upper Pliocene that is also developed at nearby Sites U1316 and U1318 (see the “Expedition 307 summary” chapter; Kano et al., 2007). The stratigraphic section was divided into two distinct units on the basis of changes in lithology. The upper lithostratigraphic Unit 1, which records the initiation and growth of Challenger Mound, consists of silty coral floatstone and rudstone. These mound-related lithologies lie atop a sharp erosional contact with sandy, sometimes glauconitic, siltstone of Unit 2, which continues to the bottom of Holes U1317A–U1317E. Sr isotopic, biostratigraphic, and magnetostratigraphic data indicate that Unit 1 formed during the last 2.6 m.y., whereas the underlying sandy siltstone (Unit 2) dates to the middle Miocene (Kano et al., 2007).

To better understand the context of mound development, two additional sites, Sites U1316 and U1318, were drilled through slope sediments that enclose Challenger Mound. Site U1316 was located upslope and Site U1318 downslope of Site U1317. The stratigraphic section at both sites was divided into three lithostratigraphic units (see the “Expedition 307 summary” chapter). At both sites, the uppermost lithostratigraphic Unit 1 consists of grayish brown silty clay with scattered pebbles interpreted by shipboard scientists as ice-rafted debris. At Site U1318, the underlying Unit 2 consists of interbedded silty clay and fine- to medium-grained sandstone with scattered pebbles. At Site U1316, Unit 2 consists of a thin (2–4 m thick) interval of silty coral floatstone interpreted on the basis of seismic data to reflect penetration of the upslope basal edge of a coral mound (see the “Site U1316” chapter). At both sites, Unit 2 unconformably overlies variably sandy and glauconitic siltstone defined as Unit 3. Seismic, stratigraphic, and biostratigraphic data show that Unit 3 at Sites U1316 and U1318 corresponds to Unit 2 at Site U1317. Units 1 and 2 at Sites U1316 and U1318 correspond to the timing of coral mound development at Site U1317 (see the “Expedition 307 summary” chapter).

Because of distinct differences in the carbon isotope compositions of skeletal carbonate, sedimentary organic matter, and microbially or thermally derived methane (Arthur et al., 1983), the δ¹³C data provided in the present study will help to constrain the major diagenetic processes affecting Challenger Mound and surrounding sediments and thereby contribute to understanding controls on the genesis of these unique ecosystems. Results are summarized in Table T1 and Figure F1.

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