Abstract

Submarine canyons are prevalent in the world’s oceans and are instrumental in transporting sediment from coastal regions to deep-sea fans. Conventional sediment parameters such as mean grain size, sorting, and provenance have typically been used to characterize these deposits, but they provide little information on sediment source or the delivery processes involved. Fortunately, transported along with the mineral grains are the remains of organisms living within the sediment. Biological constituents have unique environmental signatures that are more precise proxies for source areas than are mineral grains alone. They may be used to identify a single biofacies deposit (SBD) due to local sediment transport or a displaced, multiple biofacies deposit (MBD) resulting from staged sediment transport or a full-canyon flushing event.

This Ascension-Monterey Canyon system study demonstrates that by using the biological constituents in marine deposits, the source areas and transport mechanisms may be identified. The 19,000 year record of core S3-15G captured hemipelagic mud interspersed with individual turbidites of sand and silt transported to the core site at lower bathyal depths (3491 m). The relative abundance of displaced benthic foraminifera was found to correlate positively with grain size, with 75% of the fauna being displaced in the cross-bedded turbiditic sand (Tc) units, 39% in the laminated turbiditic sand (Td) units, and 15% in the turbiditic mud (Tet) units. Nineteen samples were SBDs representing local canyon wall sloughing, bioerosion, or hemipelagic deposition at or near the core site. Sixty-five were MBDs, 31 of which were turbiditic sands originating in the estuarine to inner shelf, outer shelf, upper slope, or upper middle slope, and the remaining 34 were turbiditic muds with displacement initiated in the estuarine to inner shelf, outer shelf, and the upper slope. Commonly, the biological remains of several biofacies characterize these MBDs, reflecting staged sediment transport with storage occurring behind slumps that act as barriers to movement until finally released. Sediment bypassing, typically of the deeper biofacies, and full-canyon flushing, were also evident. Identifying and interpreting the distribution of allochthonous biological sediment constituents in marine deposits is a powerful tool in the investigation of sediment transport that can be applied to other submarine canyon systems.

INTRODUCTION

Submarine canyons are found along the slopes of most continental margins and play an important role in transporting sediment to deep-sea fans (Garfield et al., 1994; Paull et al., 2002). Off central California, more than 400,000 m3 of sand and organic-rich material are carried from the littoral zone into Monterey Canyon each year (Paull et al., 2002). Sediment displacement may be localized (e.g., restricted to the upper portion of the canyon) and may occur several times annually when triggered by small events such as storm waves, increased terrigenous input from local rivers during peak discharge, breaking of internal waves along the continental margin, bioerosion, anthropogenic dumping of dredge material near the head of the canyon, and canyon wall slumping (Southard and Cacchione, 1972; Normark et al., 1980; Piper and Normark, 1983; Greene and Hicks, 1990; Xu et al., 2004; Johnson et al., 2006; Wain et al., 2013). At other times, deposits may be the result of “staged” transport because the sediment does not flow freely downslope, accumulating behind slumps until released by an external force that breaks the dam (Greene and Hicks, 1990; Greene et al., 2002). In rare instances, exceptionally large triggers such as earthquakes, intense storm disturbances, and catastrophic failure of canyon walls may result in complete canyon and fan channel flushing events that pass out of the upper canyon and extend across the fan (Normark and Gutmacher, 1988; Garfield et al., 1994; Johnson et al., 2001; Greene et al., 2002; Paull et al., 2002; Piper and Normark, 2009).

The trail of sand down the axis of Monterey Canyon, as well as the large crescent-shaped bedforms within it (Paull et al., 2010), suggests high energy conditions presently exist there and that the channel does, indeed, serve as the major conduit for sediment transport from the continental shelf to Monterey Fan (Greene and Hicks, 1990; Greene et al., 2002; Fildani and Normark, 2004; Paull et al., 2005). This sediment transport pathway has been documented by the movement of bottom-deployed acoustic transponders and instrument frames (Garfield et al., 1994; Paull et al., 2002, 2010). One such event impacted Monterey Canyon for at least 1000 m (from 290 m to 1297 m water depth), moving an instrument package 550 m vertically down-canyon in less than 10 min (Paull et al., 2002); another displaced a transponder 1.9 km horizontally down the canyon axis and 38 m vertically (Garfield et al., 1994). In situ measurements of sediment flows by instruments attached to moorings along the axis of the canyon recorded a maximum along-canyon velocity at 190 cm/s (Xu et al., 2004).

Off the central California coast and elsewhere around the world, turbidites are thought to be the primary mechanism responsible for sediment transport in submarine canyons and are known to occur frequently. In the northeastern Pacific, their estimated average recurrence interval is seven years in Eel Canyon (Paull et al., 2014), 34 years in Mendocino Channel (Goldfinger et al., 2012), and from 125 to 590 years in Klamath, Rougue, Smith, and Astoria Canyons (Adams, 1990; Goldfinger et al., 2012). In Monterey Canyon, they occur, on average, about every 230 years (Johnson et al., 2006).

It is often assumed that turbidity currents originate in the upper canyons on the slope below the shelf break where enough relief exists to set the sediment in motion when the continental shelf can no longer accommodate the sediment supply and the sediment must move down-canyon (Paull et al., 2005; Piper and Normark, 2009; Covault, 2011). However, the initiation sites can be highly variable, and it is difficult to locate the actual source with any degree of precision from lithology alone. Fortunately, the presence of microfaunal remains, specifically those of benthic foraminifera, among the mineral grains can aid in the identification of the source of the displaced sediments.

Benthic foraminifera are ubiquitous in the marine realm, and their distribution reflects the unique environmental conditions in which the species reside, including water mass characteristics (Zalesny, 1959; Bergen and O’Neil, 1979), sedimentology (Lankford and Phleger, 1973; Echols and Armentrout, 1980; Quinterno and Gardner, 1987; McGann, 2002), and availability of oxygen and organic matter flux (Altenbach and Sarnthein, 1989; Gooday and Turley, 1990; Jorissen, 1999). If the microfaunal distributional patterns (i.e., biofacies) of a region are known, it is possible to identify the source of displaced sediments in downslope deep-sea deposits by the character of their allochthonous fauna. Despite the simplicity of this method, few studies have investigated the distribution of foraminifera in submarine canyon and fan environments and related them to sediment transport pathways (Jorissen et al., 1994; Swallow and Culver, 1999; Schmiedl et al., 2000; Hess et al., 2005; Hess and Jorissen, 2009; Duros et al., 2011, 2013, 2014; Bolliet et al., 2014). Off central California, only Brunner and Normark (1985) and Brunner and Ledbetter (1987, 1989) have examined the foraminiferal assemblages of turbidite and hemipelagic sequences within the same cores. The purpose of this study, therefore, is to gain new insights into the delivery processes of terrigenous material to submarine fans using the biological component entrained in those sediments.

REGIONAL SETTING

Bill Normark was one of only a handful of people responsible for much of the early work characterizing canyon systems throughout the world. He utilized seismic data, seafloor observations, and coring to investigate the morphology and development of submarine channels, as well as the transport of sediment to distal fans. Today, those who are curious about such marine structures are indebted to his painstaking effort to describe their features and the processes responsible for their formation.

One of the sites Bill investigated extensively during his career was the Ascension-Monterey Canyon system located off central California (Fig. 1). The Ascension Canyon system is the more northerly of the two and is composed of Ascension, Año Nuevo, and Cabrillo Canyons (Greene and Hicks, 1990). Initially, this canyon system was the main conduit for sediment transport to the fan (Normark and Hess, 1980; Normark et al., 1983). However, it is thought to be inactive during high stands of sea level because it heads on the upper slope. In contrast, the Monterey Canyon system, composed of Soquel, Monterey, and Carmel Canyons (Greene and Hicks, 1990), transects the entire continental shelf, enabling much of it to remain active despite changes in sea level, especially during the Holocene transgression and highstand (Normark and Hess, 1980; Normark et al., 1980; Greene and Hicks, 1990; Paull et al., 2005; Fildani et al., 2006; Piper and Normark, 2009).

The sands of Monterey Canyon are thought to be derived from the beach and shelf of Monterey Bay and are generally restricted to the channel except for rare overbank transport, whereas muds representing lower-energy conditions are found on the canyon flanks (Paull et al., 2005). The Monterey Canyon system, therefore, has the characteristics of a “Type 1” submarine canyon (Jobe et al., 2011), with a coarse-grained sediment supply, indented shelf edge, fill composed of sand, a large submarine fan downslope, and domination by mass wasting and sandy turbidity currents. Yet, it is unique among those canyon systems with slightly concave profiles in that it developed across the California transform margin (Covault et al., 2011). In contrast, the presence of fine-grained sediment in upper Soquel Canyon indicates that it, like Ascension Canyon, is inactive as a conduit for the transportation of coarse sediment except during times of lower sea level (Paull et al., 2005, 2011).

Monterey Fan is one of several submarine fans off central California resulting from the deposition of Miocene- to Holocene-aged hemipelagic and turbidity current-derived sediments at the base of one or more submarine canyons (Normark, 1970a, 1999; Normark and Hess, 1980; Normark et al., 1983; Fildani et al., 1999). Presently, Monterey Fan has an area of active fan growth that extends more than 300 km from the base of the central California continental slope (Normark and Hess, 1980). This deep-sea turbiditic deposit constitutes the largest submarine fan off California (Normark, 1970a, 1999; Hess and Normark, 1976; Normark et al., 1983; EEZ-SCAN 84 Scientific Staff, 1986) and is one of the largest found offshore of the contiguous United States (Greene and Hicks, 1990).

An interpretation of Monterey Fan morphology and evolution presented by Fildani et al. (1999), Normark (1999), and Fildani and Normark (2004) suggests that the upper (proximal) portion of the fan consists of a channel-levee complex (the Upper Turbidite Sequence or UTS; ca. 0.5 Ma in age), which is spread over a 104 km2 area and whose overbank deposits dominate the modern fan. Downslope of the UTS is another series of channel, overbank, and lobe deposits (the Lower Turbidite Sequence or LTS; Fildani et al., 1999; Fildani and Normark, 2004; ca. 25 Ma in age). Below this are two lower lobes (Oldest and Abandoned; Fildani et al., 1999).

In addition to the UTS channel-levee complex, the Monterey Fan Valley is characterized by a Holocene slump (Hess et al., 1979), large-scale scour features along the Monterey East Channel (Fildani et al., 2006), and an abrupt channel meander (Shepard, 1966). The latter marks the site of probable Late Pleistocene channel diversion resulting in the abandonment of the Monterey East Fan Valley, piracy of the lower end of the Ascension Fan Valley, and subsequent headward erosion of the Ascension Fan Valley into a hanging tributary of the Monterey Fan Valley (Normark, 1970a, 1970b). Consequently, the primary source of sediments for Monterey Fan in the Quaternary is Monterey Canyon (Normark et al., 1983; Paull et al., 2005).

Both the Monterey and Ascension Fan Valleys are characterized by extensive levee development (Normark, 1970a, 1970b; Hess and Normark, 1976; Normark et al., 1983). The western levee of the Monterey Fan Valley is the largest and joins that of the Ascension Fan Valley below their convergence at a depth of 3290 m (Normark, 1970b; Greene and Hicks, 1990). The backside of this levee, away from the channel floor, is marked by the presence of sediment waves trending subparallel to the levee crest (Normark et al., 1980, 1983). These sediment waves are considered depositional bedforms resulting from channel overflow of fine-grained material of large turbidity currents (Normark et al., 1980).

Gravity core S3-15G, 4.72 m in length, was recovered by the R/V Sea Sounder (U.S. Geological Survey [USGS] cruise S-3-78-SC) 18 km from the crest of the western levee of the Monterey Fan Valley (36°23.53′N, 123°20.52′W; Fig. 1). It was obtained at a depth of 3491 m, ∼200 m below the confluence of the Ascension Fan Valley and Monterey Fan Valley (Greene and Hicks, 1990), ∼135 km southwest of Santa Cruz. The core site is presently located below the local calcium carbonate compensation depth (CCD), as evidenced by the extensive dissolution of foraminifera in the core-top sediments of this and surrounding cores (Brunner and Normark, 1985).

CORE LITHOLOGY

S3-15G is a mud-dominated deep-sea core, consisting of hemipelagic mud (Tep; Bouma, 1962) interspersed with overbank turbiditic mud (Tet) and fine-grained sand deposits. The hemipelagic and turbiditic muds are easily distinguished based upon color, appearing lighter and darker, respectively, as is often seen in deep-sea deposits (Howell and Normark, 1982; Piper and Normark, 1983; Brunner and Normark, 1985; Normark et al., 1997). The turbiditic sands are typically deposited in laminated sequences comparable to Bouma’s (1962) Td depositional division; cross-bedded (Tc) turbiditic units occur only rarely. The basal members of the Bouma cycle (Tab) are missing.

The upper 215 cm of the core is composed primarily of hemipelagic muds with occasional turbiditic muds. Turbiditic muds are more common between 215 and 370 cm, especially below 280 cm. From 370 cm to the base of the core at 472 cm, silts and sand more commonly punctuate the hemipelagic and turbiditic mud deposits. The turbiditic fine-grained silts and sands are, for the most part, laterally continuous across the diameter of the core from 0 to 370 cm, becoming more fragmented with greater core depth.

METHODS

Two techniques were used to provide a chronology for core S3-15G. The first was the temporal variation in the coiling ratio of the planktic foraminifer Neogloboquadrina pachyderma (Ehrenberg). The coiling ratio varies in relation to sea surface temperatures with right-coiling (dextral) forms dominating warm water assemblages and left-coiling (sinistral) morphotypes dominating assemblages in cool waters (Kennett, 1968; Keller, 1978; Kennett and Srinivasan, 1980, 1983), thereby providing a sensitive faunal indicator of climate in the eastern Pacific (Mix et al., 1999).

The second method used to determine the age of these hemipelagic and overbank deposits was the radiocarbon measurement of three samples of mixed planktic foraminiferal assemblages by accelerator mass spectrometry (AMS) performed by the Lawrence Livermore National Laboratory Center for Accelerator Mass Spectrometry (CAMS) facility. Ages were calculated using the accepted half-life of 14C of 5568 years (Stuiver and Polach, 1977). The original radiocarbon ages were obtained by a 14C/12C ratio and then were converted to calibrated calendar ages (calibrated years; cal yr B.P.) using the CALIB 7.0.1 program (Stuiver and Reimer, 1993; Stuiver et al., 2005; http://calib.qub.ac.uk/calib/calib.html). A reservoir age of 800 years was used for the planktic foraminiferal sample with a radiocarbon age younger than 12,000 years (Southon et al., 1990; Kienast and McKay, 2001) and an 1100 year reservoir age for the two samples older than 12,100 years (Kovanen and Easterbrook, 2002).

Ninety-two samples from core S3-15G were analyzed for benthic foraminifera. Each sample, consisting of 10–20 cm3 of sediment, was acquired from discrete lithologic units (muds or sands) within the core. The bulk samples were immersed in a dilute solution of sodium hexametaphosphate buffered to a neutral pH with ammonium hydroxide and left overnight. Following disaggregation, the samples were sieved through nested 1.0 mm, 150 µm, and 63 µm screens and then air dried. Foraminifera were extracted from the sediment >150 µm. If the residue contained abundant foraminifera, it was split into an aliquot containing ∼300 foraminifera with the aid of a microsplitter before picking; if fewer than 300 were present, all specimens were picked. These benthic foraminifera were mounted on faunal slides and identified. The slides and residues are on file at the U.S. Geological Survey, Menlo Park, California.

Benthic foraminiferal faunal counts of only 84 of the 92 samples were used in this study because eight samples (six muds and two sands) were characterized by such depauperate faunas (<125 specimens) that they were not considered statistically valid (Douglas, 1973). Species richness was determined by counting the number of species per sample, and relative species abundances were computed using a sum of total benthic foraminifera. Once the species counts were converted to frequency data, a Q-mode cluster analysis was used to describe the relationship between the benthic foraminiferal assemblages. The samples were clustered by a square-root transformation of the data, a Bray-Curtis similarity coefficient, and were amalgamated by a group-averaged linkage strategy. These methods were chosen because they treat all species equally while providing the most realistic grouping of the samples by depth (Clarke and Gorley, 2006). Primer v. 6.1.6, a statistical software package created by Primer-E, Ltd., was used for the cluster analyses (Clarke and Gorley, 2006).

In order to determine the presence of allochthonous benthic foraminifera in core S3-15G and their source, the constituents of in situ faunas were tabulated from nearly 40 studies off central California (Fig. 2) spanning the intertidal to slope regions. The regional distributions in the intertidal and nearshore environments were conducted by Cooper (1961) and Lankford and Phleger (1973). North of San Francisco Bay, specific site studies include those in the vicinity of the Russian River (Quinterno and Gardner, 1987), Bolinas Lagoon (Hedman, 1975), Tomales Bay (Bush, 1930; Mauer, 1968; Erskian and Lipps, 1987; McCormick et al., 1994; and McGann, 2008a), Cordell Bank (McGann and Powell, 2011; McGann, 1990, personal observ.), the Farallon Islands (McGann, 1990, personal observ.), and near Mulberry Seamount (Morin, 1971). Within and outside of San Francisco Bay, the faunal distributions are based on the studies of Hanna and Church (1927), McDonald and Diediker (1930), Bandy (1953a), Means (1965), Slater (1965), Quinterno (1968), Locke (1971), Connor (1975), Wagner (1978), Arnal et al. (1980), Sloan (1995, personal commun.), McGann (2008b), and McGann et al. (2013). South of San Francisco Bay, the study areas include the intertidal (Martin, 1930, 1931, 1932) and shelf (Hanna and Church, 1928; Galliher, 1932; Hanna et al., 1945; Stinemeyer and Reiter, 1958; McGann, 2002) of Monterey Bay, Monterey Canyon (Bertics, 2004; McGann, 1994 and 1999, personal observ.), Monterey Fan (McGann, 1990), the oxygen-minimum zone off Monterey (Vercoutere, 1984; Mullins et al., 1985), and the Point Sur Pinnacles (McGann, 1997). Based on the compilation of benthic foraminiferal depth distributions from these studies, and supplemented by the landmark biofacies assignments of benthic foraminifera in the northeastern Pacific of Ingle (1980) that were applied when local (particularly deep-water) studies were lacking, the faunas off central California were assigned to six biofacies that can be used to identify the source of the displaced sediments (Fig. 3). Following Ingle (1980), these six biofacies are: inner shelf, outer shelf, upper bathyal, upper middle bathyal, lower middle bathyal, and lower bathyal.

RESULTS

Sediment Age

The temporal variation in the coiling ratio of the planktic foraminifer Neogloboquadrina pachyderma (Ehrenberg) in core S3-15G suggests a few major climatic shifts have occurred off central California during the past several millennia (Fig. 4; McGann and Brunner, 1988; McGann, 1990). From 450 to 295 cm, dominance by left-coiling morphotypes is indicative of cool waters associated with the Pleistocene (Mix et al., 1999). A shift from predominantly left- to right-coiling forms beginning at 295 cm signals the onset of the Bølling-Allerød event (14,600–12,900 cal yr B.P.; Grootes and Stuvier, 1997). This is followed by a period of climatic deterioration (the Younger Dryas event, 12,900–11,600 cal yr B.P.; Grootes and Stuvier, 1997), as seen in the increase of left-coiling forms from 256 to 243 cm. A return to dominance of right-coiling forms signals the beginning of the climatic amelioration of the Holocene. The Pleistocene–Holocene boundary, dated at 11,600 cal yr B.P. (Grootes and Stuvier, 1997), lies between 241 and 212 cm in this core. For the purposes of this study, the midpoint of this interval (226.5 cm) is used as the Pleistocene–Holocene boundary in core S3-15G.

The age of the sediment recovered in core S3-15G was also determined by three radiocarbon measurements (Table 1). The sample at 450–448 cm provided a calibrated age of 19,333 cal yr B.P. for the deepest core sediments that had enough foraminifera for dating. The two additional 14C dates at 364–360 cm and 243.5–241.5 cm were calibrated at 16,405 and 12,333 cal yr B.P., respectively. All three of these ages were in stratigraphic order and corroborate the ages of the climatic events suggested by the variation in the Neogloboquadrina pachyderma coiling ratios.

Lithology and Benthic Foraminiferal Faunas

The sediments of deep-sea core S3-15G are hemipelagic muds separated by 65 turbidities of silt and sand. Twice as many of these turbidites occur in the Pleistocene sediments as in those deposited during the Holocene.

A total of 140 species of benthic foraminifera were identified in core S3-15G (McGann, 1990). Many of the taxa typically live in the deep ocean at lower bathyal depths (Ingle, 1980; Fig. 3), consistent with the depth of the core site. However, at least 19 are shallow-water species that live on the shelf or upper slope off central California (Bandy, 1953a; Quinterno and Gardner, 1987; McGann, 2002; McGann et al., 2013).

Benthic foraminiferal species richness and frequency abundance of the displaced species vary with lithology and age of the sediments (Table 2). The turbiditic sand deposits (Td + Tc) are characterized by an increase in species richness (mean = 41 specimens/sample) compared to that of the muds (Tep and Tet; mean = 34 specimens/sample), and an average of 38% of the benthic foraminifera of the turbiditic sand samples are displaced shallow-water species compared to an average of only 11% of the mud samples. In addition, the average benthic foraminiferal species richness is slightly higher in both the Holocene muds and sands (37 and 45 specimens/sample, respectively) than in the Pleistocene samples (32 and 40 specimens/mud and sand samples, respectively). The Pleistocene muds are characterized by an enrichment in allochthonous benthic foraminifera (13%) compared to their Holocene counterparts (8%), whereas the opposite is true of the sands (Pleistocene = 37%; Holocene = 41%).

The Q-mode cluster analysis of the samples segregated them into three groups (Fig. 5; Tables 3 and 4). Cluster A primarily combined Holocene muds, Cluster B grouped only Pleistocene muds, and Cluster C joined the Holocene and Pleistocene turbiditic sands.

DISCUSSION

Downslope Displacement

The movement of organisms, especially microfauna, from where they originally lived to elsewhere is commonplace in marine sediments (Natland and Kuenen, 1951; Bandy and Arnal, 1960; Kheradpir, 1970; Sliter and Baker, 1972; Merrill and Guber, 1982; Jorissen et al., 1994; Swallow and Culver, 1999; Schmiedl et al., 2000; Hess et al., 2005), particularly in regions of substantial bathymetric relief such as in submarine canyons (Phleger, 1951; Bandy, 1953b; Phleger et al., 1953; Arnal, 1976; Garfield et al., 1994; Jorissen et al., 1995; Paull et al., 2002; Hess et al., 2005). Mixing of foraminiferal faunas most often results from downslope transport of sediments by slides, slumps, debris flows, or turbidity currents (Natland and Kuenen, 1951; Bandy, 1953b, 1964; Phleger et al., 1953; Middleton and Hampton, 1973; Douglas and Heitman, 1979; Bock, 1982; Almagor, 1986; Swallow and Culver, 1999).

Coarser-grained, sand-dominated turbidity flows are estimated to travel between 7 and 22 m/sec (Komar, 1969, 1970, 1977; Krause et al., 1970). Despite the fact that foraminifera are equivalent in size to sand-sized particles (Phleger et al., 1953; Sandifer, 1969), their traction and settling velocities are lower than those of mineral grains of the same size so they only move on the order of cm/sec (Berger and Piper, 1972; Brush and Brush, 1972; Pettijohn, 1975; Kontrovitz et al., 1978, 1979; Zhang et al., 1993). Consequently, foraminiferal tests entrained within these flows will have little bearing on the outcome of deposition. Instead, breakage of tests and preferential selection of the more robust morphotypes may become a factor in determining the character of these displaced foraminiferal assemblages (Duros et al., 2014). In contrast, fine-grained turbidity flows move at speeds of ∼10 cm/sec (Normark et al., 1980). Although these speeds are low enough that they are comparable to foraminiferal traction and settling velocities, foraminiferal tests will still behave differently than terrigenous particles during transport (Stow et al., 1983), being selectively entrained within turbidity flows and differentially deposited in the overbank deposits due to their size, weight, and shape-dependent hydraulic behaviors (Berger and Piper, 1972; Kontrovitz et al., 1978, 1979; Kontrovitz and Snyder, 1981; Brunner and Normark, 1985; Brunner and Ledbetter, 1987). As a result, microfaunal assemblages of turbiditic and hemipelagic deposits are distinct (Brunner and Normark, 1985; Brunner and Ledbetter, 1987, 1989), and faunal displacement is likely in turbiditic deposits whether they are coarse or fine grained, being discerned by high species richness and the presence of allochthonous foraminifera within a sample (Natland and Kuenen, 1951; Bandy, 1953b; Bandy and Arnal, 1960; Arnal, 1976; Brunner and Normark, 1985; Schmiedl et al., 2000).

Q-mode Cluster Analysis

In order to determine the variability, source, and ecological significance of the benthic foraminiferal faunas recovered in core S3-15G, a Q-mode cluster analysis was utilized. The samples were grouped into three clusters (A–C; Fig. 5), reflecting the prevalence of the autochthonous fauna, abundance and source of the displaced specimens in the sediments, and the benthic foraminiferal faunal adaptation to changing climatic conditions in the Quaternary that distinguishes Pleistocene from Holocene samples.

Cluster A combined 12 hemipelagic and 15 turbiditic muds of Holocene age, as well as four latest Pleistocene muds (three hemipelagic and one turbiditic) and two turbiditic sands, one Holocene and the other latest Pleistocene (Fig. 5; Table 3). The frequency abundance of displaced species in the Holocene hemipelagic muds is typically low, averaging 3%, and species richness is among the lowest of all samples, averaging 32 species/sample. These parameters reflect the fact that these samples represent the final deposition site, being characterized almost exclusively by an autochthonous fauna endemic to the lower bathyal zone (Fig. 3). All of the latest Pleistocene samples that also were grouped here have few displaced species as well (2%–6%) and may actually be Holocene in age. The age uncertainty is due to: (1) the paucity of radiocarbon-dated samples constraining the Pleistocene/Holocene boundary; and/or (2) possible asynchroneity of the assemblage variations in response to the climatic change at the Pleistocene/Holocene boundary between the surface-dwelling planktic assemblage used for dating and the deep-water benthic foraminifera (McGann and Brunner, 1987). In contrast, the turbiditic muds in Cluster A contain faunal elements from three or four biofacies (inner slope to the upper middle bathyal; Table 3), reflected in higher species richness (40 species/sample) and more common displaced species (14%; Table 2). Similarly, faunas representing three biofacies with 9%–13% displaced species occur in the two turbiditic sands that clustered here (Table 4).

Cluster B combined four hemipelagic and 18 turbiditic muds, as well as nine turbiditic sands, all of Pleistocene age (Fig. 5; Tables 3 and 4). The abundance of displaced species and species richness in these hemipelagic muds is still low (4% and 30 species/sample, respectively; Table 2), reflecting, like the Holocene hemipelagic muds, the prevalence of the autochthonous lower bathyal fauna. A significant shift occurs in the turbiditic muds, however, with a substantial increase in displaced species (21%) representing commonly three, and sometimes even four, biofacies (Table 3). Faunal elements from the outer slope, upper bathyal, and upper middle bathyal biofacies were recovered in these lower bathyal fine-grained Pleistocene sediments. Similarly, an average of 25% of the fauna in the nine laminated turbiditic sand deposits (Td) grouped in this cluster were displaced, with from three to five biofacies represented (Table 4). Some of these included faunal elements from as shallow as the shelf as well as the slope; others were only from the slope.

Cluster C combined 18 laminated (Td) and two cross-bedded (Tc) turbiditic sands (Fig. 5; Table 4). The laminated sands occur in both the Holocene and Pleistocene sediments and exhibit an increase in displaced fauna (39%) and species richness (42 species/sample; Table 2) compared to those recovered in the muds, with faunal elements from two to six biofacies present. In contrast, the two cross-bedded sands are only Pleistocene in age and are characterized by an assemblage with 75% allochthonous specimens. One has faunal elements from three biofacies (upper bathyal, upper middle bathyal, and lower bathyal), whereas the other has five biofacies represented (inner shelf to lower bathyal, excluding the lower middle bathyal).

Turbiditic deposits are far more abundant in the Pleistocene portion (472–226 cm) of the core than the Holocene portion (226–0 cm), and the Pleistocene mud deposits contain a greater relative abundance of displaced specimens than do their Holocene counterparts (Table 2). Studies of Quaternary deep-sea cores obtained in the southern California Bight have reported similar findings (Bandy, 1964; Kheradpir, 1970). This Pleistocene enrichment in allochthonous foraminifera may be attributable to heightened turbidity current activity during the Pleistocene due to deposition directly on the continental slope during times of lowered sea level (Kulm and Nelson, 1967; Gorsline et al., 1968; Hess and Normark, 1976; Nelson, 1976; Stow et al., 1983), or may be related to the disturbance of sediments by the transgression that followed.

In core S3-15G, the percentage of displaced foraminifera increases with progressively larger grain size (Table 2): hemipelagic muds averaged 3%, turbiditic muds 15%, laminated turbiditic sands 39%, and cross-bedded turbiditic sands 75%. Brunner and Normark (1985) also found a high percentage of displaced foraminifera in a study of ten turbidites from three cores from the western levee of Monterey Fan. In their study, basal Tcd layers contained more than 30% allochthonous shallow-water species, whereas they defined hemipelagic samples as having no displaced fauna and samples with 1%–19% displaced fauna as difficult to classify. Other studies have also shown that transported coarser-grained sediments are commonly characterized by higher relative abundance of displaced benthic foraminifera than are finer-grained deposits (Phleger, 1951; Bandy, 1964).

Foraminiferal species diversity generally increased with grain size in core S3-15G as well (Table 2), as the muds averaged 31 and 35 species/sample in the hemipelagic and turbiditic deposits, respectively, whereas the sands averaged 37 and 44 species/sample, with the cross-bedded sands slightly lower than the laminated sands. The higher diversity in the sands may result from one or more factors: particularly favorable ecological conditions for foraminifera at the source of the turbidity currents (Hessler and Sanders, 1967; Sanders and Hessler, 1969; Douglas and Woodruff, 1981), infaunal versus epifaunal habitat (Loubere, 1989; Loubere and Gary, 1990), a reduction in the effects of dissolution due to an increase in the speed of deposition, and/or the addition of displaced species to the endemic deep-water fauna at the final deposition site (Sliter and Baker, 1972; Ingle, 1980; Brunner and Normark, 1985; Brunner and Ledbetter, 1987). The latter is considered the most important factor in core S3-15G, illustrated by the large number of turbidites that incorporated faunal elements from several biofacies (Fig. 6).

Single and Multiple Biofacies Deposits and Mechanisms of Transport

The biological constituents entrained in sediment can provide information not only on where the sediment was derived but also possible mechanisms and the extent of transport. Sediment containing benthic foraminifera of only one biofacies, here defined as a single biofacies deposit (SBD), would be characteristic of very restricted transport near the sampling site resulting from small, localized events such as storm waves, peak river discharge, breaking of internal waves, bioerosion, canyon wall slumping, or simply the deposition of hemipelagic mud at a site (Fig. 7). The latter three processes could be responsible for the deposition of the 19 hemipelagic mud samples in S3-15G as they only contained a lower bathyal benthic fauna typical of the 3491 m core site (Fig. 6), whereas the others are more likely shelfal processes that would have introduced foraminifera characteristic of the continental margin. These local transport events are thought to occur on a subannual basis (Xu et al., 2004; Johnson et al., 2006), the timing of which is corroborated by the Stevens et al. (2013) study of coupled optically stimulated luminescence (OSL) ages of quartz sand deposits and benthic foraminiferal 14C ages in Monterey Canyon, which determined that sand is transported locally through the upper canyon on an order of tens of years and the lower canyon from 150 to 250 years. With such rapid recurrence intervals and limited spatial impact, the localized movement of sediment most likely involves SBDs with sediments of similar age.

In contrast, a multiple biofacies deposit (MBD) contains benthic microfauna of more than one biofacies, with the associated sediment potentially including a range of grain sizes. A MBD would result from staged sediment transport, with multiple episodes of this storage-and-release process occurring sequentially so as to move the sediment progressively down the channel (Fig. 7). The complexity of this staged sediment transport is evident in S3-15G, where 65 MBDs were deposited over a span of 19,000 years. Of the 34 turbiditic muds, origination sites varied from the estuarine to inner shelf down to the upper slope, whereas in the 31 turbiditic sands, displacement was initiated in the estuarine to inner shelf, outer shelf, upper slope, or upper middle slope (Fig. 6). There is also substantial evidence that the MBDs accumulated benthic foraminifera as they moved down the canyon. The turbiditic muds were characterized by microfauna of two to three biofacies upslope of the lower bathyal core site, and the turbiditic sands contained one to five upslope biofacies. Finally, sediment bypass is apparent in both the turbiditic muds and sands; many MBDs did not incorporate the deeper biofacies as they traveled to the core site (Fig. 6).

The biological evidence for staged sediment transport illustrated in core S3-15G is also supported by other studies nearby. The Monterey Canyon OSL study of Stevens et al. (2013) showed that sand typically moves down the canyon in multiple events instead of one large event and that the sand is temporarily stored in upper and middle canyons for 10s to 1000s of years. Similarly, differences in the age of foraminifera from three biofacies within a single turbidite recovered at lower bathyal depths (2658 m) farther north off central California in Eel Canyon most likely reflect staged sediment storage and transport: inner shelf and upper middle bathyal biofacies differ in age by ∼525 years, whereas those typically residing at the shelf break were ∼6300–6830 years older (see Supplementary Table 2 of Paull et al., 2014). Both of these examples demonstrate that caution should be used when dating turbidites because a MBD may contain biofacies of different ages.

In extreme events such as earthquakes, intense storms, or catastrophic canyon wall failures, complete canyon and fan channel flushing occurs. A MBD may be deposited as a result because sediment transport triggered by these major disturbances typically flows across many biofacies (Fig. 7). Despite the fact that these events are fairly rare compared to staged transport, some of the MBDs in S3-15G may represent full-canyon flushing events, especially those that captured all, or nearly all, of the biofacies as they traveled to the final deposition site (Fig. 6). Whereas in a recently deposited turbidite or even a sediment trap where the presence of living and dead foraminifera could relatively easily distinguish staged sediment transport versus a full-canyon flushing event, a historic sediment record such as that of S3-15G would require the costly technique of dating individual biofacies (e.g., Eel Canyon cited above) within a turbidite in order to distinguish the two. Clearly though, it is not difficult to determine the extent of transport (local versus widespread) from the biological constituents entrained in the sediment.

CONCLUSIONS

A gravity core obtained from the western levee of Monterey Fan contains hemipelagic muds interspersed with 65 Pleistocene and Holocene turbiditic overbank deposits. Twice as many of these turbidites occur in the Pleistocene portion of the core as in the Holocene section and are assumed to reflect greater turbidity current activity induced by low Pleistocene sea level and the subsequent transgression.

The hemipelagic and turbiditic muds are characterized by a small percentage of displaced foraminiferal tests and the lowest species richness of the core. The muds are dominated by a lower bathyal foraminiferal assemblage endemic to the core site. In contrast, the turbiditic sands originated on the shelf to the upper middle slope. They are distinguished by an extremely diverse benthic foraminiferal assemblage and a large proportion of allochthonous benthic foraminifera, resulting from the addition of the displaced shallow-water dwelling species to the deep-water fauna living at the deposition site and a possible reduction in dissolution due to rapid burial. Thus, the relative abundance of displaced foraminifera correlates positively with grain size and with greater sediment age.

Core S3-15G provides a 19,000 year record of local (SBD) deposition punctuated with MBD turbidites. The entrained benthic foraminifera can help identify the source of these turbidite flows and the delivery system, whether local, staged, or full-canyon flushing, responsible for transporting sediment down the canyon and eventually to the fan.

This study would not have been possible had the late William Normark (U.S. Geological Survey) not made core S3-15G available for study. He was always such a positive force and mentor to many, and he is sorely missed in the hallways of the USGS. I also wish to thank Charlotte Brunner (University of Southern Mississippi) for assisting in the design and supervision of this project. Both Charlotte and Christina Gutmacher (formerly USGS) are also acknowledged for aiding in the core description. Kristin McDougall-Reid (USGS) kindly shared her unpublished foraminiferal data off central California, and Krystle Anderson (Monterey Bay Aquarium Research Institute) generously provided the location map. Thanks to the captain and crew of the R/V Sea Sounder (USGS) for recovering the Monterey Fan core that was used in this study. Radiocarbon dates were provided by the Lawrence Livermore National Laboratory Center for Accelerator Mass Spectrometry. This manuscript greatly benefited by the reviews of Charles Powell, II, John Barron (USGS), and two additional anonymous reviewers.