Research Paper GEOSPHERE Modern and ancient hiatuses in the pelagic caps of Pacific guyots and seamounts and internal tides GEOSPHERE; v. 11, no. 5 Neil C. Mitchell1, Harper L. Simmons2, and Caroline H. Lear3 1School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK doi:10.1130/GES00999.1 2School of Fisheries and Ocean Sciences, University of Alaska-Fairbanks, 905 N. Koyukuk Drive, 129 O’Neill Building, Fairbanks, Alaska 99775, USA 3School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK 10 figures CORRESPONDENCE: neil .mitchell@ manchester ABSTRACT landmasses were different. Furthermore, the maximum current is commonly .ac .uk more important locally than the mean current for resuspension and transport Incidences of nondeposition or erosion at the modern seabed and hiatuses of particles and thus for influencing the sedimentary record. The amplitudes CITATION: Mitchell, N.C., Simmons, H.L., and Lear, C.H., 2015, Modern and ancient hiatuses in the within the pelagic caps of guyots and seamounts are evaluated along with of current oscillations should therefore be of interest to paleoceanography, al- pelagic caps of Pacific guyots and seamounts and paleotemperature and physiographic information to speculate on the charac- though they are not well known for the geological past. internal tides: Geosphere, v. 11, no. 5, p. 1590–1606, ter of late Cenozoic internal tidal waves in the upper Pacific Ocean. Drill-core Hiatuses in pelagic sediments of the deep abyssal ocean floor have been doi:10.1130/GES00999.1. and seismic reflection data are used to classify sediment at the drill sites as interpreted from sediment cores (Barron and Keller, 1982; Keller and Barron, having been accumulating or eroding or not being deposited in the recent 1983; Moore et al., 1978). Hiatuses are formed by either physical removal of Received 11 November 2013 Revision received 2 May 2015 geological past. When those classified sites are compared against predictions material by currents or dissolution of soluble components of the sediment. Accepted 23 June 2015 of a numerical model of the modern internal tidal wave field (Simmons, 2008), They can be difficult to recognize, being complicated by incomplete recovery Published online 13 August 2015 the sites accumulating particles over the past few million years are found to of stratigraphy by coring and requiring missing biostratigraphic zones to be be away from beams of the modeled internal tide, while those that have not identified (Moore et al., 1978). For pragmatic reasons we use low accumula- been accumulating are in internal tide beams. Given the correspondence to tion rates (condensed sections) as signs of breaks in sedimentation, which are the modern internal wave field, we examine whether internal tides can ex- loosely referred to as hiatuses, although they may not strictly speaking be hia- plain ancient hiatuses at the drill sites. For example, late Cenozoic pelagic tuses in the sense originally intended. Individual hiatuses have been attributed caps on guyots among the Marshall Islands contain two hiatuses of broadly to changes in deep-ocean circulation originating from tectonic changes of similar age, but the dates of the first pelagic sediments deposited following gateways and changes in production of deep water at high latitudes (Barron each hiatus do not correlate between guyots, suggesting that they originate and Keller, 1982; Keller and Barron, 1983). The presence of microfossils out of not from ocean chemical changes, but from physical processes, such as ero- stratigraphic sequence is evidence that hiatuses can be at least partly caused sion by internal tidal waves. We investigate how changing conditions such as by physical movements (Thiede, 1981). Barron and Keller (1982) noted that Mio- ocean temperature and basin physiography may have affected internal tides cene hiatuses correlate with sea-level lowering, greater 18O in benthic forami- through the Cenozoic. Allowing for subsequent rotation or uplift by plate tec- nifera, and cool faunal assemblages, consistent with intensified deep-water tonics, ancient submarine ridges among the Solomon, Bonin, and Mariana production and circulation. However, Moore (2013) argued that the general island chains may have been responsible for some sediment hiatuses at these circulation was likely too sluggish to explain reworked radiolaria around the distant guyot sites. Eocene-Oligocene boundary in deep-water samples, suggesting that higher frequency motions such as from internal waves may have been involved. The sediments deposited in the summits of guyots and seamounts poten- INTRODUCTION tially record changing physical conditions within the upper ocean. Watkins et al. (1995) noticed that pelagic cap thicknesses on guyots decline going north Within the subject of paleoceanography, much effort has been expended from the equator, diminishing to zero at ~25°N, a trend also found in seis- investigating the ocean structure of large-scale currents from paleontolog- mic data (van Waasbergen and Winterer, 1993). Watkins et al. (1995) identified ical, isotopic, and geochemical evidence. Much less is known about water erosion as the cause of the thinning, although the origin of the erosion was movements of shorter periods, such as those associated with eddies, baro- unknown at that time. Pearson (1995) developed a higher resolution biostratig- tropic tides, internal waves, and surface waves, although these movements raphy of samples from three drill sites on guyots among the Marshall Islands; For permission to copy, contact Copyright may have been significantly different in the geological past, for example, be- he identified hiatuses and noted that they terminated at different times, hinting Permissions, GSA, or [email protected]. cause wind and temperature regimes and physiography of the oceans and that they had a physical rather than chemical origin. Pearson (1995) suggested © 2015 Geological Society of America GEOSPHERE | Volume 11 | Number 5 Mitchell et al. | Internal tidal beams and sediment hiatuses on guyots Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/5/1590/3337169/1590.pdf 1590 by guest on 24 September 2021 Research Paper that pelagic cap development proceeds from an early barren stage to first ac- cumulation under the influence of currents, allowing sediments to accumulate only in the center of the platform, followed by progressive expansion of that 883,1203,1204 deposit to the platform edges under more quiescent conditions. Detroit Increasing sophistication of physical oceanographic models, combined Emperor Seamount with more accurate estimates of basin shape (Smith and Sandwell, 1997) and 433 Suiko 0123 ocean density structure, have allowed increasingly more accurate estimates 431 Depth (km) to be made of physical conditions at the modern seabed. In addition, the Geo- 432,1205 40˚ MapApp software tool (www .geomapapp .org) has provided easy access to a 430 Ojin revised consistent stratigraphy of the drill cores recovered by the Deep Sea s 308,1206 879 Drilling Project (DSDP) and Ocean Drilling Program (ODP) (Fig. 1). This combi- nation now allows an exploration of the spatial correspondence between inter- nal tidal wave propagation across the Pacific predicted by numerical modeling 878 MIT D (Simmons, 2008) (Fig. 2) with modern seabed hiatuses. Although the model C A and the geologic data represent markedly different time scales, we suggest 866 Resolution B reasons why the internal wave field may not have changed greatly over the 20˚ 865 Allison Hawai’i 44 Horizon past few million years. Furthermore, assessing the presence or absence of hia- 171 Mid-Pacific Mountains tuses using seismic and drill-core data together effectively provides estimates 200 Ita Mai Tai 202 873 Wodejebato over large spatial and temporal scales, which can be preferable to overcome 0 872 Lo-En 2 D local variability likely to affect individual shallow cores. Given the modern cor- 4 0 871 Limalok 2 C respondence, we speculate on earlier conditions in the late Cenozoic upper Marshall 0 4 ) Islands ) 2 ocean. Although it is not possible to infer the ancient pattern of the internal 4 C 0˚ 0 2 B wave field from the limited sedimentary information, we highlight some tec- 0 4 2 * B tonic movements that might have affected the wave field and might explain Depth (km 4 0 Depth (km 2 some hiatuses. 0 2 4 A * A This issue is complex, so we devote the following section to describing the 4 0 0220000 404000600600800 nature of internal waves (how they are generated and propagate) and other Distance (km (km) ) flow phenomena over seamounts and guyots that may also affect sediment 140˚ 160˚ 180˚ -160˚ deposits. We then introduce the data sets and provide an interpretation of them, bearing in mind the physical oceanographic results. In the discussion, Figure 1. Bathymetry of the central-west Pacific Ocean, highlighting 0–3 km depths relevant to the origins of the earlier hiatuses are considered speculatively in terms of evi- internal wave generation. Black areas represent depths >3 km and land areas (islands outlined in white). Numbers are scientific drilling sites; those with large white circles are guyot or sea- dence for changing water temperatures and changing basin physiography, mount sites with superior seismic and sample data for assessing internal wave effects. Large which affected the conversion of barotropic tides into internal waves. circles are some