Abyssal Pacific Research Articles

Statistical characteristics of the Northwestern Pacific abyssal irregularities and their potential impact on the tsunami propagation

A set of echograms obtained during the course of the Pacific Transects project in the second half of 1980 was digitized with a similar step within a distance of 1 mile. Based on the processing of these materials, the spectral and statistical characteristics of the irregularities of the bottom topography of the Northwestern Pacific were obtained. It was shown that the decrease of the spectral density within the wavelength range from 2 to 100 miles is well consistent with the k−2 law with the subdivision of the spectrum into two ranges: the small-scale (2–8 miles long and 50 m high) and medium-scale (10–40 miles long and 120 m high) ones. The dampening decrement of the tsunami wave due to the interaction with the irregularities of the bottom topography accounted for γ = 0.2 hr−1 for the forward scattering and 0.1 hr−1 for the back scattering. The forward scattering provides the transformation of the initial single pulse into the wave train, the delay of the maximum wave with respect to the arrival time of the tsunami, and the increase in the duration of the oscillations.

Oceanic tracer and proxy time scales revisited

Abstract. Quantifying time-responses of the ocean to tracer input is important to the interpretation of paleodata from sediment cores – because surface-injected tracers do not instantaneously spread throughout the ocean. To obtain insights into the time response, a computationally efficient state-transition matrix method is demonstrated and used to compute successive states of passive tracer concentrations in the global ocean. Times to equilibrium exceed a thousand years for regions of the global ocean outside of the injection and convective areas and concentration gradients give time-lags from hundreds to thousands of years between the Atlantic and Pacific abyss, depending on the injection region and the nature of the boundary conditions employed. Equilibrium times can be much longer than radiocarbon ages – both because the latter are strongly biased towards the youngest fraction of fluid captured in a sample, and because they represent distinct physical properties. Use of different boundary conditions – concentration, or flux – produces varying response times, with the latter depending directly upon pulse duration. With pulses, the sometimes very different transient approach to equilibrium in various parts of the ocean generates event identification problems.

Isotopic evidence for the source of lead in the North Pacific abyssal water

The absence of accurate measurements of lead (Pb) isotopic composition in the North Pacific abyssal water has made it difficult to assess the relative importance of what are believed to be the two major Pb sources: the natural Pb introduced during preindustrial time and recent anthropogenic Pb resulting from leaded gasoline combustion and high temperature industrial activities. Here we report a vertical profile of seawater 206Pb/ 207Pb ratio and a meridional section of Pb concentration in the North Pacific Ocean. We observe 2–3-fold increases in Pb concentration along the deep-water flow path and a deep-water 206Pb/ 207Pb ratio (∼1.188) substantially lower than the pre-industrial value (∼1.210). These data suggest that anthropogenic Pb has invaded the North Pacific abyssal water and become the predominant Pb source there. A simple model calculation based on these data indicates that the anthropogenic Pb is transported to the deep ocean by sinking particles and that this Pb vertical flux has a 206Pb/ 207Pb ratio that decreased during the past two centuries.

Carbon dioxide release from the North Pacific abyss during the last deglaciation

Atmospheric carbon dioxide concentrations were significantly lower during glacial periods than during intervening interglacial periods, but the mechanisms responsible for this difference remain uncertain. Many recent explanations call on greater carbon storage in a poorly ventilated deep ocean during glacial periods, but direct evidence regarding the ventilation and respired carbon content of the glacial deep ocean is sparse and often equivocal. Here we present sedimentary geochemical records from sites spanning the deep subarctic Pacific that--together with previously published results--show that a poorly ventilated water mass containing a high concentration of respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum. Despite an inferred increase in deep Southern Ocean ventilation during the first step of the deglaciation (18,000-15,000 years ago), we find no evidence for improved ventilation in the abyssal subarctic Pacific until a rapid transition approximately 14,600 years ago: this change was accompanied by an acceleration of export production from the surface waters above but only a small increase in atmospheric carbon dioxide concentration. We speculate that these changes were mechanistically linked to a roughly coeval increase in deep water formation in the North Atlantic, which flushed respired carbon dioxide from northern abyssal waters, but also increased the supply of nutrients to the upper ocean, leading to greater carbon dioxide sequestration at mid-depths and stalling the rise of atmospheric carbon dioxide concentrations. Our findings are qualitatively consistent with hypotheses invoking a deglacial flushing of respired carbon dioxide from an isolated, deep ocean reservoir, but suggest that the reservoir may have been released in stages, as vigorous deep water ventilation switched between North Atlantic and Southern Ocean source regions.

Influence of surface texture and microhabitat heterogeneity in structuring nodule faunal communities

Manganese nodules provide an important habitat for sessile benthic organisms, many of them novel foraminifera, in the abyssal Pacific. The surface texture and microhabitat heterogeneity of nodules may play a significant role in structuring these communities. We analyzed the distribution of foraminiferal species on 20 nodules collected using either a USNEL box corer or the Nautile submersible sampling tool at a 5000-m-deep site in the Tropical North Pacific. The nodules had an upper region characterized by two microhabitats, namely raised surfaces and intervening depressed surfaces. The more or less vertical sides of the nodules were regarded as a third microhabitat. The upper region had a predominantly smooth texture while the sides had a rough texture. We tested whether the percent cover of each of the 34 species examined was distributed evenly over the three microhabitats and the two surface textures (smooth vs rough). More than half (20 out of 34) of species covered a greater area on smooth surfaces than on rough surfaces, while one-third of the species (12 out of 34) occurred more on raised microhabitats than on depressed surfaces and nodule sides. These two results are closely interrelated, since 78% of the raised surfaces were characterized by a smooth texture. The nodule-encrusting species were feeding either on suspended particles or on particles deposited on the nodule surfaces. Because most species had a mat-like or domed morphology, we suspect that nodule surface feeding was more prevalent. Current flow, which transports propagules and particulate food, may be the main environmental factor explaining the observed faunal distributions, as it is in the case of other hard-substrate assemblages.

Biodiversity of nematode assemblages from the region of the Clarion-Clipperton Fracture Zone, an area of commercial mining interest

BackgroundThe possibility for commercial mining of deep-sea manganese nodules is currently under exploration in the abyssal Clarion-Clipperton Fracture Zone. Nematodes have potential for biomonitoring of the impact of commercial activity but the natural biodiversity is unknown. We investigate the feasibility of nematodes as biomonitoring organisms and give information about their natural biodiversity.ResultsThe taxonomic composition (at family to genus level) of the nematode fauna in the abyssal Pacific is similar, but not identical to, the North Atlantic. Given the immature state of marine nematode taxonomy, it is not possible to comment on the commonality or otherwise of species between oceans. The between basin differences do not appear to be directly linked to current ecological factors. The abyssal Pacific region (including the Fracture Zone) could be divided into two biodiversity subregions that conform to variations in the linked factors of flux to the benthos and of sedimentary characteristics. Richer biodiversity is associated with areas of known phytodetritus input and higher organic-carbon flux. Despite high reported sample diversity, estimated regional diversity is less than 400 species.ConclusionThe estimated regional diversity of the CCFZ is a tractable figure for biomonitoring of commercial activities in this region using marine nematodes, despite the immature taxonomy (i.e. most marine species have not been described) of the group. However, nematode ecology is in dire need of further study.

Importance of the eastern Indian Ocean for the abyssal Pacific

The role of the eastern Indian Ocean in controlling the abyssal circulation in the Pacific is examined using a 1° × 1° world ocean model. Thanks to the improvement of bottom topography and the incorporation of a bottom boundary layer model, the bottom water formation off the Adélie Coast is produced. This bottom water is not only the source of the abyssal water in the eastern Indian but also has a great influence on the whole abyssal Pacific. The lower part of Circumpolar Deep Water (CDW), which enters the abyssal Pacific passing through south of New Zealand, changes its property through mixing with this bottom water flowing underneath CDW in the Australian‐Antarctic Basin and the South Australia Basin. The tracer experiments show that the water ventilated off the Adélie Coast occupies about 20% of the abyssal Pacific, whereas North Atlantic Deep Water (NADW) occupies about 50%, the Weddell Sea bottom water about 20%, and the Ross Sea bottom water about 10%. The flux of the ventilated water estimated by the tracer experiments is 17.1 Sv for NADW, 6.3 Sv from the Weddell Sea, 6.2 Sv from off the Adélie Coast, and 3.5 Sv from Ross Sea in this model. The total flux of 33.1 Sv is not far from the estimate from observations, though it may overestimate by 1–4 Sv. This does not directly indicate that the real world ocean should have the large amount of the bottom water sources in the Pacific‐Indian sector. However, the model offers a picture of the water properties consistent with the observation.

Polychaete species diversity in the central Pacific abyss: local and regional patterns, and relationships with productivity

We investigated the relationship between productivity and local species diversity, and the degree of species turnover, at 8 sites on the central equatorial Pacific abyssal plain. The 8 sites span a 4-fold difference in seafloor particulate organic carbon (POC) flux and, hence, community productivity. The sites are similar in water depth (4300 to 5100 m), degree of isolation from terrigenous influences, and hydrodynamic regime. Three sites lie under the influence of equatorial upwelling, and are subject to enhanced deep POC flux derived from high overlying primary production. The remaining sites lie beneath the oligotrophic north Pacific gyre. The number of polychaete species collected at a single site ranged from 14 to 113, with at least 90% apparently being new to science. We found no evidence for the purported unimodal relationship between productivity and diversity seen in other ecosystems, including deep-sea slopes, and found only weak evidence of a monotonic increase in diversity with productivity. Rates of species turnover were low over scales of ~200 to 3000 km for the dominant polychaete species in the communities, and all sites were dominated by a core group of biogeographically widespread, locally abundant species. In contrast, there was little between-site similarity in the long list of rare species found at each site, implying either a high turnover of rare species at 200 to 3000 km scales, or incomplete sampling of the rare species list at each site. More intensive sampling studies using both morphological and molecular methods are needed to resolve the distribution patterns of rare species in the Pacific abyss. Local polychaete species diversity beneath equatorial Pacific upwelling (measured by rarefaction) appears to be unusually high for the deep sea, exceeding by at least 10 to 20% that measured in abyssal sites in the Atlantic and Pacific, and on the continental slopes of the North Atlantic, North Pacific, and Indian Oceans.

Pelagic—benthic coupling in the abyssal eastern North Pacific: An 8‐year time‐series study of food supply and demand

An 8‐yr time‐series study of the trophic coupling between a pelagic food supply and its utilization by the sediment community was conducted at 4,100‐m depth in the eastern North Pacific between 1989 and 1998. Supply of sinking particulate organic carbon (POC) and particulate total nitrogen (PTN) was estimated from collections made with sediment traps moored at 3,500‐ and 4,050‐m depth (600 and 50 m above bottom). Sediment community oxygen consumption (SCOC), an estimate of aerobic utilization of organic matter, was measured in situ. POC and PTN fluxes at both depths declined significantly from October 1989 through 1996 then increased in 1998. Organic carbon : total nitrogen (C : N) of the sinking particulate matter fluctuated ~10 throughout the study, except for a major peak (84.8) at 50 m above bottom in summer 1995, indicating lateral advection of organic material of terrestrial origin. POC: SCOC declined progressively over the first 7 yr, from a high of 0.99 in 1989–1990 to 0.22 in 1995–1996. In 1998, there was an increase in POC: SCOC to 0.43, suggesting that the food deficit was reduced by an increase in sinking flux. A continuing deficit in food supply cannot be sustained without ultimately affecting the structural and functional characteristics of the sediment community. The importance of undersampling by sediment traps, lateral advection of marine and terrestrial organic matter from the continental slope and shelf, and dissolved organic matter is evaluated. These other sources of nutrients for benthic organisms may be important in explaining some of the observed discrepancy between supply and demand in the abyssal Pacific. Decadal‐scale climate variations that influence marine primary production and terrestrial discharges into the ocean may be extremely important in understanding biogeochemical processes in the deep sea.

The distribution of radionuclides and some trace metals in the water columns of the Japan and Bonin trenches

Presented here is the first detailed geochemical data set on the U/Th series Th, Pa, Ac, and Pb isotopes and artificial fallout radionuclides ( 90Sr, 137Cs, and Pu isotopes), and some trace elements (V, Zn, Cd, Cu, Mn, and Ni) in two water columns of the Japan and Bonin trenches down to the bottom depths of 7585 m and 9750 m, respectively. Hydrographic properties such as temperature, salinity, dissolved oxygen, and nutrient content within the trench valley remain constant at the same levels as those in the bottom water of the Northwest Pacific basin (typically ∼6000 m in depth). The radionuclide activities and most trace metal concentrations are also not very different from those in the overlying water at depths of around 5000–6000 m. This means that any chemical alteration which sea water undergoes during its residence within the trench was not obviously detected by the techniques used here. The suggestion follows that the trench water is rather freely communicating by isopycnal mixing with the bottom water overlying the Northwest Pacific abyssal plain. The trench waters contain high 239, 240Pu activities throughout, indicating that Pu is actively regenerating from rapidly sinking, large particles at the bottom interface, probably due to a change in the oxidation state. On the other hand, the vertical profiles of 210Pb and 231Pa show lower activities within the trench than those in the overlying deep waters, suggesting that the effect of boundary and bottom scavenging is significant in controlling their oceanic distributions. However, none of the trace metals studied here obviously follows the behaviour of the above nuclides. The 228Th data show scattering within the Bonin Trench that is largely ascribable to analytical errors. If, however we accept that the scatter of 228Th data is real and the variation is caused solely by decay of its parent 228Ra, we can set an upper limit of ∼5 years for the renewal time of the trench water.

Distributions of dissolved organic and inorganic carbon and radiocarbon in the eastern North Pacific continental margin

Temporal variations in the natural radiocarbon ( 14C) signatures of dissolved organic and inorganic carbon (DOC and DIC, respectively) in seawater have been studied previously (Druffel, E.R.M., Bauer, J.E., Williams, P.M., Griffin, S., Wolgast, D.M., 1996. Seasonal variability of radiocarbon in particulate organic carbon in the northeast Pacific. J. Geophys. Res. 101, 20 543–20 552; Bauer, J.E., Druffel, E.R.M., Williams, P.M., Wolgast, D.M., Griffin, S., 1998. Temporal variability in dissolved organic carbon and radiocarbon in the eastern North Pacific Ocean. J. Geophys. Res. 103, 2867–2882) at a long-term time-series station (Sta. M: 32°N, 123W) in the eastern North Pacific located at the eastern edge of the North Pacific abyssal plain. In June 1995 a transect was made from Sta. M inshore to approximately 500 m depth in order to evaluate the distributions of 14C in DOC and DIC from the abyssal plain to the upper continental slope. Concentrations and Δ 14C values of DOC in mixed layer waters (25 and 85 m) decreased toward the upper slope. In deeper waters, concentrations and Δ 14C values were in general similar at all three sites. Differences in DOC concentrations and Δ 14C-DOC between Sta. M and the rise and upper slope sites were explained in part by the mixing of DOC and Δ 14C along constant density ( σ t) surfaces. However, specific deviations from conservative behavior due to mixing were observed for Δ 14C-DOC at mesopelagic (∼700 m) and near-bottom (∼3600– 3900 m) depths of the continental rise. Comparable findings are reported for DIC, where σ t-normalized concentrations and Δ 14C values in Sta. M, rise and upper slope waters were similar, with the exception of slight increases in concentrations and Δ 14C values in near-bottom waters of the rise. These observations indicate that both DOC and DIC in continental rise and slope surface waters of the eastern North Pacific Ocean margin are comprised of a component of actively upwelled material derived from deeper offshore waters and a component of recent surface-derived material. Our data suggest that contributions of both 14C-enriched and 14C-depleted DOC and DIC to localized regions of the water column occur in waters of the continental rise and slope. The sources of material having anomalous Δ 14C values are not certain, but may include organic matter derived from off-shelf and slope transport, particulate organic carbon solubilization, and sediment porewaters. We also examine the relationships between Δ 14C-DOC and both Δ 14C-DIC and Δ 14C of suspended POC measured in a companion study as a means of evaluating similarities or differences in the mechanisms responsible for the distributions of each at all three sites.

Abyssal currents generated by diffusion and geothermal heating over rises

A continuously stratified (in both salinity and temperature) diffusive time-dependent one-dimensional f-plane model over a sloping bottom is constructed. The model is used to investigate the role of mixing of density near the bottom on large-scale abyssal flow near mid-ocean rises. For realistic abyssal values, both geothermal heating from the bottom and diffusion can be important to the dynamics of flow over mid-ocean rises. When diffusion dominates, buoyancy is transported toward the bottom and the θ- S (potential temperature-salinity) relation remains nearly linear. When geothermal heating dominates, the θ- S relation hooks near the bottom and a convectively driven mixed layer forms. Both effects reduce the density and stratification near the bottom. In contrast, bottom-intensified diffusion has the same effect near the bottom but results in an increase of density and stratification some distance above the bottom. If the bottom slopes, a horizontal density gradient results, setting up a geostrophic, bottom-intensified, along-slope flow that can effect mass transport. Evidence of the importance of these processes is found in the abyssal Pacific. Just over the western flank of the East Pacific Rise, a 700–900 m thick layer of low N 2 (buoyancy frequency) is warmer, saltier, and lighter than interior water at the same depth. This layer is described with CTD data from recent hydrographic sections at nominal latitudes 15°S and 10°N. If the interior is motionless, this low N 2 layer transports 4 and 8 × 10 6 m 3 s −1 equatorward above the western flank of the rise at 15°S and 10°N, respectively. This equatorward current, a direct result of diffusion and heating over a sloping sea-floor, has a volume transport comparable to those of the deep western boundary current at these latitudes.

Rapid and frequent turbidite accumulation in the bottom of Izu-Ogasawara Trench: Chemical and radiochemical evidence

Two sediment cores (pilot gravity and piston) were obtained from the bottom of the Izu-Ogasawara Trench at 9750 m and analyzed for various elements and radioisotopes. The results showed a history of complex and frequent turbidite deposition: In the gravity core, eight layers rich in manganese were observed, of which five are enriched in Cu and Co as well. The other three are also enriched in Mo but no other heavy metals, suggesting the presence of at least two mechanisms of formation. Trapping of iron manganese micronodules can account for the enrichment of Mn, Cu and Co. The other three layers rich in Mn and Mo appear to be formed by a post-depositional diagenetic process of Mn mobilization and redeposition in the sediment column. A strong correlation between 226Ra and Cu in the gravity core suggests that the 226Ra was also carried into the bottom of the trench in turbidites in association with Mn micronodules. Little excess of 210Pb over 226Ra was found at the top but the excess was significant at mid-depths from 30 to 70 cm, indicating that those sediments were deposited within the last 200 y. In the piston core there is a sharp discontinuity of chemical and radiochemical composition around a depth of 250 cm. Below that depth the sediments appear to be dominated by materials derived from terrestrial sources, as compared with those in the upper layer which are of contemporary marine origin. 226Ra is deficient relative to 230Th throughout the sediment column down to ∼ 6 m. This finding is consistent with the finding that the average rate of sediment accumulation is 1–2 orders of magnitude faster than that in the western North Pacific abyssal plain, suggesting the convergence of materials into the bottom of the trench.

Analysis of growth zones and microstructure in otoliths of two macrourids from the North Pacific abyss

The microstructure of the sagittae of Coryphaenoides armatus variabilis and C. yaquinae from the North Pacific abyss was studied in thin sections with light microscopy. Growth increments fitting the general description of daily growth increments known from sagittae of shallow-living fishes (e.g., hake) were observed and numerically related to larger-scale ring patterns. These larger-scale patterns included rings which appeared to be annual (i.e., tertiary rings) and those of an unknown periodicity (i.e., secondary rings). The ratio of the number of increments to tertiary rings ranged between 181:1 and 406: 1 in C. a. variabilis. These values essentially agree with observed ratios between increments and yearly annuli of hake sagittae. It is suggested that large abyssal macrourids may be relatively young and thus have extraordinarily rapid rates of growth.

The trans-Pacific spread of equatorial shallow-marine benthos in the Cretaceous

Abstract From Aptian to Maastrichtian times, the development of broad tracts of anomalously shallow ocean floor in the Pacific, studded with reef-rimmed seamounts and islands, allowed some Caribbean benthos to spread westwards by larval drift between staging posts in Tethyan/Atlantic waters debouching between the Americas. There is as yet no compelling evidence that the Pacific was completely traversed in this manner during the first and major phase of intra-plate volcanothermal activity in the Aptian-Albian: evidently there still existed a sufficiently wide region of Pacific abyss to the west to serve as a barrier to larval dispersal. However, clear evidence for the ‘landfall’ of Caribbean-derived species on eastern Tethyan shores in Campanian to Maastrichtian times is provided by several examples of disjunct endemics from the two regions. Such taxa include rudist bivalves, gastropods, larger foraminifera, and calcareous algae. This episode coincides with a second, smaller phase of mid-plate volcanothermal activity in the Pacific, which, in conjunction with the relict topography from the earlier phase, evidently furnished a temporarily continuous chain of staging posts for the trans-Pacific spread of the shallow-marine benthos.

Giant Amphipod from the Abyssal Pacific Ocean

A baited camera has recorded a lysianassid amphipod that is twice as large as the largest amphipod previously recorded. The locality for this mobile omnivore is the sterile bottom of the eastern North Pacific Ocean, at a depth of 5304 meters.

Ecology and Zoogeography of some Polychaeta Representatives of the Abyssal Pacific

Intensive oceanographic research during the past 20 years has given new data on the distribution of the deep-sea bottom fauna, thus making possible the establishment of basic distribution laws. These new data on the composition and distribution of the deep-sea benthos have been used to study the relationship of both the whole bottom fauna and its separate groups to the peculiar environmental conditions of the abyssal ocean. In particular, recent work on polychaetes has shown the existence of a specific abyssal fauna among this most widely distributed oceanic group

A Wandering Enteropneust from the Abyssal Pacific, and the Distribution of "Spiral" Tracks on the Sea Floor

Certain coiled tracks appear in photographs from the bottom of most oceans and are abundant in some regions. Enteropneusts are among the forms responsible for such tracks although, despite earlier evidence, they are rarely considered to be either active or abyssal.