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Since the first publications of subseafloor microbial abundances (Cragg et al., 1990, 1992; Parkes et al., 1994), cell abundance was generally assumed to decrease logarithmically by about three orders of magnitude with sediment depth over the upper 1000 meters below seafloor (mbsf). It was postulated that there is very little variability between sites (review in Parkes et al., 2000). Most of these studies were carried out on sediments from ocean margins and the eastern equatorial Pacific Ocean. Cell count data from Ocean Drilling Program (ODP) Leg 201 provided a first indication that subseafloor microbial cell abundance has at least some positive correlation with primary productivity of the overlying ocean (D’Hondt, Jørgensen, Miller, et al., 2003; D’Hondt et al., 2004). Drill sites in more oligotrophic oceanic regions (ODP Sites 1225 and 1226) revealed cell abundance profiles that had the same general trend but were shifted toward lower values, whereas sites from high-productivity areas (Sites 1229 and 1230) showed higher than average cell abundances.

Cell counts from the South Pacific Gyre (D’Hondt et al., 2009), the most oligotrophic oceanic region on Earth, provided further evidence for the observed positive correlation between productivity of the surface water and cell abundance; at the same subseafloor depth cell abundances were three to four orders of magnitude lower than any other previously published data set. Cell counts also showed a much stronger decrease with depth (approximately three orders of magnitude over the upper 1 mbsf). Despite the deviation in total cell abundance and rate of decrease with depth, the South Pacific Gyre data still show the same general trend, a logarithmic decrease with depth. Kallmeyer et al. (2012) used published data sets and their own cell count data to revise the estimate of global subseafloor microbial biomass (Whitman et al., 1998). Their study proposes not only much lower cell numbers but also an even lower biomass because of the assumption of smaller cell sizes. According to their study, cell numbers decrease with depth according to a power-law function and subseafloor microbial biomass is largely controlled by distance to shore and sedimentation rate.

Burial of photosynthetically derived organic matter from the photic zone is considered to be the primary source of electron donors for microbes in most sedimentary subseafloor environments (D’Hondt et al., 2004; Jørgensen, 2000). Studies that use very different approaches (e.g., quantification of intact phospholipids [Lipp et al., 2008] and pore water oxygen profiles [Røy et al., 2012]) indicate that organic matter availability strongly controls microbial activity and abundance. The rate of organic matter oxidation in subseafloor sediment has been described as declining with age according to a power-law function (Middelburg, 1989) or logarithmically (Rothman and Forney, 2007), similar to many cell count records.

Cell count data from several sites, however, do not follow the general trend of a smooth power-law decline with depth. Such deviations have multiple causes. Elevated temperature acting over geological time will lead to kinetically controlled abiotic organic maturation reactions converting recalcitrant organic matter such as kerogen into volatile products like fatty acids and petroleum that can migrate over long distances and supply microbial communities with organic substrates at cold sites like gas hydrates (Mangelsdorf et al., 2005) or cold seeps in the deep sea (Joye et al., 2004). Horsfield et al. (2006) showed that in areas with high heat flow, like the Nankai Trough off Japan, in situ microbes utilize thermogenically produced substrates because temperatures become sufficiently high at relatively shallow depths. Locations with an in situ coupling will most probably remain rare because despite reports about microbes being able to survive at temperatures well over 100°C (Blöchl et al., 1997; Kashefi and Lovley, 2003; Takai et al., 2008), at temperatures above 80°C microbial activity is severely reduced and sediment becomes basically pasteurized (Wilhelms et al., 2001). Given the relatively low temperatures in the sediment retrieved during Integrated Ocean Drilling Program (IODP) Expedition 320/321, thermogenic generation of microbial substrates can be ruled out.

Discrete layers of high organic matter content (e.g., sapropels) can also cause positive excursions in microbial abundance profiles (Cragg et al., 1999). Coolen et al. (2002) showed that Pleistocene sapropels in sediment of the Mediterranean Sea are still sites of enhanced microbial activity and abundance, despite their age and the low temperatures in the sediment. Apparent even at low temperatures, sufficient amounts of microbial substrates are produced from largely recalcitrant organic matter that was buried millions of years ago (Petsch et al., 2001).

Not just electron donors in the form of sedimentary organic carbon are controlling microbial abundance and activity in subseafloor sediments, but electron acceptors as well. In cases where deep-seated brines are present, electron acceptors like sulfate can diffuse upward and cause reverse geochemical gradients, leading to an increase in microbial activity (D’Hondt et al., 2004) and abundance (Cragg et al., 1999).

The equatorial Pacific upwelling is a narrow band of high productivity within 2° latitude of the Equator. The upwelling zone stretches from the west coast of the American continent westward for several thousand kilometers through the Pacific Ocean. High productivity in the surface water also leads to higher sedimentation rates and higher organic matter content in the deposited sediment. Sediment that was deposited inside the equatorial upwelling zone therefore have a different composition than those from outside the upwelling zone.

Throughout the Cenozoic, the northwestward movement of the Pacific plate has had a northward latitudinal component of ~0.25°/Ma, transporting equatorial sediment gradually through the zone of highest sediment delivery. By moving the equatorial sediment out of the zone of high sedimentation, excessive burial beneath younger sediment is prevented. This phenomenon allowed Expedition 320/321, Pacific Equatorial Age Transect (PEAT), to recover a continuous Cenozoic record of the paleoequatorial Pacific.

Eight sites (U1331–U1338) were visited to obtain samples of sediment from the equatorial upwelling zone at a paleoposition of successive crustal ages on the Pacific plate.