Previous   |    Next

doi:10.2204/iodp.sp.341.2011

Introduction

Global climate during the Neogene to Quaternary is distinguished by the transition into a colder, more variable world dominated by the onset and intensification of major Northern Hemisphere glaciations (Zachos et al., 2001). Corresponding to this alteration is a generally acknowledged global increase in sediment accumulation in both continental and marine sedimentary basins (Donnelly, 1982; Molnar and England, 1990; Zhang et al., 2001; Hay et al., 2002; Molnar, 2004; Willett, 2010), which is attributed to increased erosion in orogenic belts related to larger-amplitude climate fluctuations (Zhang et al., 2001; Willett, 2010). For many orogenic settings, this increased erosion may be driven by the expansion of alpine ice, but the direct correlation between increased erosion and specific climate drivers is lacking. Yet, the consequences of this increased erosion are potentially far-reaching. Worldwide analyses of orogenic belts (Koons, 1995; Pinter and Brandon, 1997; Pavlis et al., 1997; Zeitler et al., 2001; Hoth et al., 2006; Roe et al., 2006; Stolar et al., 2006; Whipple, 2009) have shown that earth systems cannot be considered to be the product of a series of distinct, decoupled tectonic and climatic processes. Rather, there is a complex interplay between deformation, exhumation, and climate systems.

Exhumation plays a key role in controlling the regional distribution of metamorphic rocks, local climate change, and development of structures throughout an orogen (Fig. F1). As tectonic processes influence regional climate by raising mountains that enhance orographic precipitation patterns and intensity, the Neogene–Quaternary climate transition likely affected tectonic processes through changes in erosion rates that redistributed mass and subsequently altered stresses in orogenic wedges (Willet, 1999; Roe et al., 2006; Whipple, 2009). Analytical models examining the coupling between glacial erosion and orogenic processes reveal that glacial erosion can significantly modify the patterns and rates of erosion in an orogenic wedge (Roe et al., 2006; Tomkin, 2007; Tomkin and Roe, 2007). Glacial climate interacts with mountain building through erosion and sediment transport, dispersal, and accumulation. At a critically tapered wedge, erosion in the inner part of an orogen results in increased thrusting in order to attempt to maintain critical taper (e.g., Berger et al., 2008a; Whipple, 2009). Deposition of the eroded sediments in the outer part of an orogen can in turn suppress deformation due to loading (e.g., Simpson, 2010; Worthington et al., 2010). A critical question is, at what stage of the deteriorating Neogene climate is an orogen ultimately driven into subcriticality? Does this lead to increased exhumation in the glaciated core of a mountain belt, enhanced topographic relief, and migration of the locus of sediment accumulation to the toes of an orogen that impacts deformation patterns?

Addressing the linkages between global climate change, modification of the dynamics of surficial processes, and subsequent tectonic responses requires integrated studies of orogenic systems in areas that exemplify specific end-members of the problem. The Gulf of Alaska borders the St. Elias orogen of Alaska and Canada, the highest coastal mountain range on Earth and the highest range in North America (Fig. F2). This orogen is <30 Ma in age, and mountain building occurred during a period of significant global climate change (Fig. F3), allowing this expedition to examine the response of an orogenic system to the establishment of a highly erosive glacial system (Hallet et al., 1996; Jaeger et al., 1998; Sheaf et al., 2003; Berger et al., 2008b; Enkelmann et al., 2010; Spotila and Berger, 2010; Finzel et al., 2011). The sediments emanating from the orogen are deposited in a relatively geographically confined area offshore, providing a rare opportunity to use the stratigraphic record to quantify spatial and temporal variations in the erosional flux from land to sea. The geological processes in southern Alaska are comparable to those observed in the Himalayan orogeny and include extremely high erosion rates, active faulting beneath mountains and alpine glaciers, and orogenesis coincident with extensive glacial cover. Important advantages of Alaska over the Himalaya include the proximity of a high coastal mountain range next to an energetic ocean with essentially no intervening basins to trap sediment. Therefore, tectonic and climatic signals have the potential to be quickly recorded in offshore areas with little modification resulting from long transport in rivers or temporary storage in intervening sedimentary basins.

Additionally, the implications of the Neogene glacial growth in the circum-North Pacific are far reaching beyond a tectonic response to increased glacial erosion and exhumation. As climate determines the timing and patterns of precipitation, it controls glacial dynamics, erosion, and sediment/meltwater fluxes to the ocean. Establishing the timing of northwestern Cordilleran ice sheet (NCIS) advance–retreat cycles will address a major challenge in Quaternary paleoclimatology, which is to know the extent to which glacial-age climate change was a synchronous worldwide event and what the driving mechanisms were for potentially propagating millennial-scale warming–cooling cycles around the globe (oceanic, atmospheric, or both) (Clapperton, 2000; Mix et al., 2001; Hill et al., 2006). Although many paleoclimate and glaciologic records provide strong evidence for millennial-scale climate change along the Gulf of Alaska margin (i.e., Last Glacial Maximum [LGM], Younger-Dryas, early Holocene Hypsithermal, late Holocene Neoglacial) (Peteet and Mann, 1994; Mann et al., 1998; Calkin et al., 2001; Davies et al., 2011), the timing and character of these variations in relation to North Atlantic or Southern Ocean records over the Pliocene–Pleistocene are largely still unknown. NCIS glaciation is fueled by low rates of evapotranspiration, extensive North Pacific moisture delivery, and extreme rates of precipitation due to the predominant storm track coupled with orographic lift along the high coastal mountain range (Royer, 1982; Emile-Geay et al., 2003; Neal et al., 2010). Melting of this ice and return of the freshwater to the modern coastal ocean results in high specific discharge, presently two− to sixfold higher than the Amazon and Congo (Neal et al., 2010). This discharge creates nearly estuarine-like salinity conditions in the coastal ocean and is a substantial contributor to the freshwater budget of the Bering Sea and the Arctic Ocean (Weingartner et al., 2005), which in turn may impact the thermohaline stability of the North Atlantic during interglacials (Keigwin and Cook, 2007). North Pacific Intermediate Water episodically forms in the Gulf of Alaska (You et al., 2000) but may have been much more significant during the last deglacial (Heinrich Event 1), potentially linked to global adjustment of thermohaline circulation and teleconnections that impact atmospheric moisture delivery to the North Pacific (Okazaki et al., 2010; Menviel et al., in press). The Gulf of Alaska/North Pacific Ocean is the largest high-nutrient–low-chlorophyll (HNLC) area in the Northern Hemisphere, and productivity in this area is largely iron-limited (Harrison et al., 1999), with shelf oceanographic processes and surface water discharge appearing to play a role in regulating surface-ocean iron concentrations (Stabeno et al., 2004; Schroth et al., 2009; Wu et al., 2009). Evidence of substantial changes in surface productivity in the Gulf of Alaska since the LGM (Davies et al., 2011; Addisson et al., submitted) indicates that millennial-scale climate change and eustasy in the northeast Pacific Ocean has a first-order effect on primary productivity. Furthermore, the modes of transfer of glacigenic sediments and the spatio-temporal variation in transfer rates are critical to deciphering the architecture of the massive (as thick as 5 km), high-latitude late Tertiary and Quaternary Northern Hemisphere continental margin sedimentary sequences (Riis, 1992; Vagnes et al., 1992; Eidvin et al., 1993; Lagoe et al., 1993; Elverhøi et al., 1995; Powell and Cooper, 2002; Dahlgren et al., 2005). These thick deposits contain a rich history of climate change recorded in both proxy climate data (e.g., iceberg-rafted debris and microfossils) and sediment accumulation rates that, in part, reflect climate-driven glacial sediment yields. Exceptionally high rates of glacigenic sediment accumulation in the undersampled northeast Pacific also allow development of a paleomagnetic record of geomagnetic field variability on submillennial scales to assess geomagnetic persistence, a signature of the mantle’s influence on the geodynamo and the paleomagnetic record (Gubbins et al., 2007; Stoner, 2009; Amit et al., 2010).

Integrated Ocean Drilling Program (IODP) Expedition 341, which combines IODP Proposal 686-Full and ancillary proposal letter (APL)-786, will investigate the northeast Pacific continental margin sedimentary record formed during orogenesis during a time of significant global climatic deterioration in the late Miocene to recent, which has led to development of the most aggressive erosion agent on the planet, a temperate glacial system. Sedimentary provenance and paleoclimatic, glacimarine, and structural sedimentary indicators tied to a multicomponent chronology will be used to generate detailed records of changes in the locus and magnitude of glacial erosion and sediment and freshwater delivery to the coastal ocean and their impact on oceanographic conditions in the Gulf of Alaska and the resulting continental margin stratigraphic record on the interaction of these processes. Additionally, drilling on the Surveyor Fan, which undergoes a mixture of subduction and accretion at the Aleutian Trench (Fig. F4), will recover a detailed Pleistocene tephra record of Aleutian arc volcanism and will aid in understanding how sediment inputs influence subduction zone processes. Because the oceanographic processes in the Gulf of Alaska directly impact the Bering Sea, Expedition 341 will strongly complement IODP Expedition 323 by addressing the late Neogene evolution of continental glaciation and freshwater and nutrient inputs, but in a more proximal source to the glacial drivers of many of these processes.

Top of page   |    Previous   |    Next