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Preservation of laminations in hemipelagic and pelagic sediments is routinely attributed to reduced oxygen conditions that inhibited benthic activity. As such, the degree of preservation of laminae is used commonly as a paleo-oxygenation indicator. Recent evidence from deep-sea pelagic sediments in the eastern equatorial Pacific and North Atlantic, however, demonstrates that preservation of laminae may occur by rapid deposition of ungrazed Thalassiothrix longissima diatom mats that overwhelm the benthos in otherwise oxic bottom waters. Here, we report that similar diatom mat laminae are common in hemipelagic sediments of the Guaymas Basin in the Gulf of California, have aided preservation of the laminated sediment sequence and have also enhanced opal burial. Examples of diatom mats influencing sediment preservation are also described from the northern California Margin and from the Miocene Monterey Formation, southern California. Thus, even in classical settings of laminated sediment occurrence, caution needs to be taken in correlating degree of lamina preservation with paleo-oxygen levels. Such studies must be underpinned by detailed microfabric and micropaleontological analyses.

It is manifest in the study of dysoxic sediments from the geological record that infaunal burrowing is considered so severely limited by the lack of dissolved oxygen as to be non-existent. Although the effects of megafauna and macrofauna on sedimentary and geochemical processes are well known, the effects of meiofauna are largely ignored. Here we document abundant meiofauna in the recent severely dysoxic, laminated sediments from the Santa Barbara basin, California margin, and also microcavities and microtunnels in laminated deglacial sediments from Palmer Deep, west Antarctic Peninsula, that we interpret to be open, relict nematode burrows. Santa Barbara basin box-core subcores were sieved to quantify metazoan abundance, and others were embedded with resin for examination of meiofaunal life positions using confocal microscopy. Metazoan densities in the surface centimeters of sediment range from 80.7 to 117.9 [cm.sup.-3], and nematode populations, together with their abundant burrows, remain quite high to at least 3 cm. Scanning electron microscope analysis of fractured surfaces in Palmer Deep sediments revealed that the rigid diatom ooze framework aids the preservation of ~50 [micro]m diameter open nematode burrows. These structures were observed to at least 40 m below the seafloor surface. This is the first description of a nematode-produced open burrow network preserved in the geological record. Optical microscopy of resin-embedded thin sections revealed widespread sediment redistribution without significant lamina disruption. The implications of abundant nematode burrows in surface sediments, and their preservation in the geological record, are wide ranging for both modern and ancient dysoxic marine environments, including for determining early sediment fabric production, geochemical processes, and diagenetic reactions in the oxic and suboxic zones. Keywords: bioturbation, Santa Barbara basin, Palmer Deep, laminations, geochemistry.