Elsevier

Marine Geology

Volume 174, Issues 1–4, 15 March 2001, Pages 249-271
Marine Geology

Organic geochemistry of Saanich Inlet, BC, during the Holocene as revealed by Ocean Drilling Program Leg 169S

This paper is dedicated to Louise M. Adamson who sailed with us on ODP Leg 169S and generously assisted with the shipboard organic geochemistry
https://doi.org/10.1016/S0025-3227(00)00154-7Get rights and content

Abstract

Sites 1033 and 1034 of ODP Leg 169S in Saanich Inlet have an unusual diagenetic system, that has the appearance of being depth reversed, i.e. a bacterial methane accumulation zone underlain by a sulphate reduction zone. During the late Pleistocene grey, undifferentiated, glacio-marine clays were deposited with low Corg contents (<0.4 wt.%), and interstitial fluids replete in SO4 (ca. 27 mM), devoid of CH4 and low in nutrients. This indicates oxic conditions are present, reflecting the open exchange of waters with Haro Strait during the Pleistocene before the Saanich Peninsula emerged. In the earliest Holocene (ca. 11,000 years BP) the inlet was formed, severely restricting water circulation, and leading to the presence of anoxic bottom waters. The sediments are laminated and show a dramatic rise to high Corg, Norg and Stot contents (up to 2.5, 0.4, 1.4 wt.%, respectively) over a period of ca. 1000 years. The nutrient concentrations are especially high (TA, NH4, PO4 up to 115 meq/l, 20 mM and 400 μM, respectively), SO4 is exhausted and CH4 is prolific. Stable carbon isotope ratio measurements of CH4 and co-existing CO2 indicate that methanogenesis is via carbonate reduction (δ13C–CH4 ca. −60 to −70‰, δ13C–CO2 ca. +10‰). At the sulphate–methane interfaces, both at the near-surface and at 50 mbsf (Site 1033) and 80 mbsf (Site 1034) methane consumption by sulphate reducing bacteria is intensive.

Introduction

On August 15, 1996, the drilling vessel JOIDES Resolution of the Ocean Drilling Program (ODP) sailed from Victoria into Saanich Inlet to occupy two Sites in the deeper central basin of Saanich Inlet (ODP Sites 169S-1033, and 169S-1034, Fig. 1b). The primary objective of ODP Leg 169S was the high resolution sampling of the finely annular laminated sediments in the anoxic basin of Saanich Inlet. Details of the drilling prospectus and preliminary results are contained in “Initial Reports of the Ocean Drilling Program Leg 169S” (Shipboard Scientific Party, 1998). In addition, other scientific results are contained in companion papers elsewhere in this volume.

The sediments and water column of Saanich Inlet have been long recognized as an excellent location to study organic geochemistry, in particular early diagenesis under anoxic conditions. As early as the 1960s, several papers dealt with the oceanographic and geochemical situation of Saanich Inlet that leads to the establishment of anoxic bottom water conditions (Gross et al., 1963, Gucluer and Gross, 1964, Buddemeier, 1969, Herlinveaux, 1962, Herlinveaux, 1966, Pickard, 1975, Juniper and Brinkhurst, 1983, Thomson, 1994). In the 1970s a series of three landmark papers, i.e. Nissenbaum et al., 1972, Presley et al., 1972, Brown et al., 1972 described the diagenetic setting in sediments of Saanich Inlet. These early papers, including those of Murray et al., 1978, Ahmed et al., 1984 established the relationships in Saanich Inlet between the accumulating organic material and nutrients released through microbially mediated remineralisation reactions.

Other early studies dealt with the distribution, production and consumption of dissolved gases in the water column and near-surface sediments of Saanich Inlet. These studies included examinations of the reduction of dissolved sulphate (SO4) to sulphide by sulphate reducing bacteria (refs), the formation and consumption of methane by methanogens and methanotrophs and hydrogen gas production (Lilley et al., 1978).

As a result of these earlier and subsequent papers, dealing with the geology (e.g. Muller, 1981, Yorath and Nasmith, 1995), geochemistry and biology of the Saanich Inlet water column and near-surface sediments, there exists a detailed database and understanding of the diagenetic situation proximal to the ODP 169S sites. These provide important background information for the ODP results presented here.

Several studies have examined the near-surface depositional, sedimentologic and geochemical history that has lead to the establishment of the ultra-high resolution laminated Holocene sediments (Powys, 1987, Heusser, 1993). At the head of Saanich Inlet a sill rises to approximately 70 m water depth (Fig. 1, Fig. 2). As a consequence of this obstruction, the circulation and renewal of oxygenated waters into the Saanich Inlet basin from the Haro Strait, across the Swanson and Satellite Channels, is severely restricted (Fig. 1b; Anderson and Devol, 1973). This present oceanographic setting, coupled with high organic loading in the water column through elevated seasonal primary productivity, leads to high oxygen utilisation in the water column. In the restricted deeper waters of Saanich Inlet, adjacent to ODP Sites 1033 and 1034, this situation creates anoxic conditions as indicated by the dissolved oxygen isopleths in Fig. 2 (qv. Stucchi and Giovando, 1983). As the anoxic remineralisation of organic material proceeds, bacterial sulphate reduction is dominant, resulting in intensive sulphide generation and accumulation in the sediments and deeper water column (Fig. 2). A consequence of the sulphide is that there is currently no significant infauna in the near-surface sediments of Saanich Inlet. Hence there is no bioturbation of the sediments beneath the anoxic bottom water package. The absence of bioturbation has resulted in an exquisite package of laminae or varves that record intra-annular information throughout the Holocene.

An example of the varved sediments spanning a time interval of ca. 70 years is shown in Fig. 3afor the core section 1033B 4H5, 76–105 cm. Each of the individual varves typically consists of a laminae triplet comprised of grey silty mud (winter deposit), and olive diatomaceous ooze (spring to early summer plankton bloom) and a darker olive grey diatomaceous mud (late summer, early fall) (Sancetta and Calvert, 1988, Sancetta, 1989, Heusser, 1993, Blais, 1992, Blais, 1995, Collins, 1997). Numerous studies have investigated primary productivity in Saanich Inlet, as the cause of the light laminae, e.g. Takahashi et al., 1978, Hobson, 1983, Hobson and McQuoid, 1997. The annual varve thickness is typically around 6 mm but varies from approximately 3 to 15 mm (Shipboard Scientific Party, 1998). The chronology for the ODP 169S has been established based on varve counting, radiometric carbon-14 dates (Fig. 4; Buddemeier, 1969, Robinson and Thompson, 1981, Bornhold et al., 1998), and by using marker horizons, such as the Mazama Ash layer at 7645 years BP (Bacon, 1983, Abella, 1988). The anoxic laminated Pleistocene sediments extend to approximately 60 mbsf at Site 1033 and ca. 80 mbsf at Site 1034. The hemipelagic laminated Holocene sediments are underlain at both sites by glacio-marine muds of late Pleistocene age (Fig. 4).

An example of these grey, largely undifferentiated muds is shown in Fig. 3b using the core photograph of 169S 1033B 9H5, 76–105 cm. The transition from glacio-marine muds to the hemipelagic laminated sediments is strongly demarcated at both coring sites, and represents a clear change in the physical, chemical and biological oceanographic conditions of Saanich Inlet at the Pleistocene–Holocene boundary. As illustrated in Fig. 1a (Huntley et al., 2001), the Saanich Peninsula prior to ca. 11,000 years BP was still submerged and had not yet rebounded isostatically through the deloading of retreating glacial ice, e.g. Holland, 1964, Alley and Chatwin, 1979, Clague et al., 1982, Bobrowsky and Clague, 1990, Blyth and Rutter, 1993, Blyth et al., 1993, Bobrowsky et al., 1993, Clague, 1994. As a consequence, during the Pleistocene there was no Saanich Inlet, per se; rather the ocean waters could flow directly in to the Inlet across the submerged peninsula from the Haro Strait. It was only at the start of the Holocene, i.e. after ca. 11,000 years BP, that the Saanich Peninsula emerged, isolating the Inlet behind the sill (near Oceanographic Station S6 in Fig. 1b). This change in circulation enhanced primary productivity in the Inlet while at the same time restricting the replenishment of oxygen, thereby establishing the anaerobic bottom waters.

Section snippets

Methods

The basic shipboard sampling and analytical methods that were used in this study are:

  • 1.

    Solid Constituents, i.e. carbonate (Ccarb), organic carbon (Corg), organic nitrogen (Norg) and total sulphur (Stot).

  • 2.

    Interstitial Fluid Nutrients, i.e. Titration Alkalinity (TA), Dissolved Ammonia (NH4), phosphate (PO4) and sulphate (SO4).

  • 3.

    Void Space Gas (VSG) and Headspace Gas composition and concentrations.

A brief description of the methods follows, however for more details the interested reader is referred to

Organic carbon and nitrogen

As seen in Fig. 5there is a major facies change at the Pleistocene–Holocene boundary at both Sites 1033 and 1034. The older, massive grey glacio-marine clay at depth is largely undifferentiated (Fig. 3b) and persists until approximately 11,000 years BP. At both drill sites this section (Unit II) contains low total organic carbon contents (Corg<0.4 wt.%, Fig. 5). Similarly, organic nitrogen in Unit II remained below 0.08 wt.% at both sites. In Unit II the total sulphur was also low, i.e. at or

Conclusions

The Saanich Inlet is an exciting example of anaerobic remineralisation reactions and organic matter decomposition. The diagenetic environment is unusual in that there is sulphate, sulphate reduction and methane oxidation in both the near-surface sediments and at depth around the Pleistocene–Holocene boundary. The diagenetic situation is largely controlled by the amount of organic matter that has been deposited in both the Pleistocene and Holocene. Methanogenesis by carbonate reduction is a

Acknowledgements

We would like to thank the master and the crew of the research vessel JOIDES Resolution for their assistance in sampling in the Saanich Inlet. We also appreciate the contributions to the paper made by Dave Stucchi (IOS) on water column chemistry and David Huntley (CEOR) on the physiography of the Saanich Inlet region. The authors greatly appreciate the efforts of shipboard scientists J. Morford, and A. Russell. Special thanks is extended to J. Gieskes and the shipboard technical personnel D.

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    Present address: Max-Planck-Institute for Marine Microbiology, Celsiusstr. 1, D-28359 Bremen, Germany. Fax: +49-421-2028-690.

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