Flux and fate of fluvial sediments leaving large islands in the East Indies

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Journal of Sea Research 41 (1999) 97–107

Flux and fate of fluvial sediments leaving large islands in the East Indies John D. Milliman a,Ł , Katherine L. Farnsworth a , Christina S. Albertin b a

School of Marine Science, College of William and Mary, Gloucester Pt, VA 23062, USA b Geology Department, College of William and Mary, Williamsburg, VA 23185, USA Received 5 March 1998; accepted 24 August 1998

Abstract Because of their generally small drainage basin areas, high topographic relief, relatively young and erodible rocks, and heavy rainfall, rivers draining the high-standing islands of the East Indies transport a disproportionately large amount of sediment to the ocean. Rivers on the islands of Sumatera (Sumatra), Jawa (Java), Borneo, Sulawesi (Celebes), Timor and New Guinea are calculated to discharge about 4:2 ð 109 t of sediment annually. Although these six islands only account for about 2% of the land area draining into the global ocean, they may be responsible for as much as 20 to 25% of the sediment export. Fluvial sediment leaving these islands is discharged into several distinctly different provinces: shallow epicontinental seas such as the Sunda Shelf, Gulf of Papua and Sea of Arafura; and narrow-shelf, active margins along the western and southern sides of Sumatra and Java, and the north coast of New Guinea. High-resolution seismic profiles in the Gulf of Papua (New Guinea) show a clinoform sequence of Holocene sediments pinching out on the mid- to outer shelf, with sediment thickness locally greater than 40 m near the coast; some — but perhaps not much — sediment escapes to the outer shelf and the deeper Papua Trough beyond. In contrast, seismic profiles off northern New Guinea show river-derived sediment prograding over and by-passing a narrow shelf that locally has buried a relict barrier reef. A small fraction of the sediment escaping the northern shelf may be transported to the eastern equatorial Pacific by way of the Equatorial Counter Current, where it may help fertilize equatorial upwelling.  1999 Elsevier Science B.V. All rights reserved. Keywords: rivers; East Indies; seismic profiles; New Guinea; sediments

1. Introduction In recent years the global importance of rivers draining southern Asia and the high-standing islands in the East Indies (also referred to as Oceania) has become increasingly evident. Milliman and Meade (1983) estimated that more than 70% of the sediment entering the oceans comes from rivers draining southern Asia and Oceania, the result of mountainŁ Corresponding

author. E-mail: [email protected]

ous terrain, erodible strata often impacted by human activities such as deforestation and agriculture (e.g., Milliman et al., 1987; Douglas, 1996), and seasonally heavy rainfall. Based on the few data available in the early 1980s, mostly from Taiwan and New Zealand, Milliman and Meade speculated the average sediment yield of New Zealand and Taiwan was 1000 t km 2 y 1 , more than 5 times greater than the global average. Accordingly, Milliman and Meade (1983) speculated that rivers draining the high-standing islands in the East Indies (primarily Papua New Guinea, In-

1385-1101/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 5 - 1 1 0 1 ( 9 8 ) 0 0 0 4 0 - 9

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donesia and the Philippines) may discharge 3 ð 109 t of sediment annually, or about 20 to 25% of the total sediment entering the global ocean. Because of the greater erosive ability of mountain rivers (e.g., Pinet and Souriau, 1988), Milliman and Meade’s numbers almost certainly underestimated the potential sediment discharge from these highstanding islands. At least one reason for this underestimation was not taking into account the fact that small rivers generally discharge much greater loads relative to their drainage basin areas than do large rivers (Milliman and Syvitski, 1992). This greater sediment yield from smaller basins results from less storage capacity and greater response to episodic events, such as floods and landslides. Because they are mountainous, small in area and numerous, East Indies rivers, according to Milliman and Syvitski (1992), may discharge as much as 9 ð 109 t, or about half of the total sediment flux to the ocean, even though collectively they drain only about 3% of the land area emptying into the global ocean. The 280 rivers listed by Milliman and Syvitski (1992) provided a sufficient data base to create predictive algorithms, such that if basin area and maximum elevation were known, the sediment load of ungauged rivers could be estimated with some degree of confidence. Assuming that the large island of New Guinea is potentially a substantial contributor of sediment to the ocean, Milliman (1995) used existing maps to determine the areas of 253 river basins on the island, estimated the maximum elevations of their headwaters, and then applied the Milliman and Syvitski algorithms to predict sediment loads. Rivers with headwaters less than 500 m in elevation were ignored since they generally carry small sediment loads compared to rivers draining higher terrain. In fact, upland rivers (maximum elevations of 500– 1000 m) also were found to have low loads, meaning that only mountainous rivers are major suppliers of sediment on New Guinea. The 253 New Guinean rivers, some of which have drainage basins less than 100 km2 in area, were calculated to discharge approximately 1:7 ð 109 t of sediment to the ocean annually (Milliman, 1995), about the same as all the rivers draining North America (Milliman and Meade, 1983). Approximately one-half of this sediment discharges to the north side of the island, which is bordered by a narrow shelf

adjacent to a deep trench that locally appears to have been buried by large amounts of land-derived sediment (Hamilton, 1979; Milliman, 1995). In contrast, rivers flowing to the south discharge most of their sediments to a broad shelf, particularly the Gulf of Papua to the east and Arafura Shelf to the west. Accordingly, most of the sediment discharged to the north probably escapes to the deep sea at present, whereas most of the sediment discharged to the south may be trapped on the inner shelf, mostly escaping during periods of lowered sea level. Given the large sediment loads escaping New Guinea, as well as the contrasting physiographic environments into which the sediment is transported, the present authors have calculated the fluvial sediment fluxes from several other large high-standing islands in the East Indies, specifically Sumatera (Sumatra), Jawa (Java), Borneo, Sulawesi (Celebes) and Timor. For the purposes of this paper we have ignored geopolitical borders. As such, we treat New Guinea as an island, not as Irian Jaya and Papua New Guinea; and we consider Borneo as a single entity, although realizing that political rule of this island is divided between Indonesia, Malaysia and Brunei. Because we are interested in the fate of these fluvial sediments as well as their flux, we present several seismic profiles recently acquired off Papua New Guinea. We also discuss how the fluvial discharge from these islands and other islands in the East Indies may be linked to open-ocean processes.

2. Calculation of sediment fluxes from the six East Indies islands To calculate the sediment loads of rivers draining the Indonesian islands (and Borneo), we used the finest-scale and most recent maps of the islands available at the US Geological Survey (Reston, VA) to determine both drainage basin area and maximum elevation of each river; generally this was at the 1 : 1 000 000 scale. We slightly modified the Milliman and Syvitski (1992) algorithms to determine sediment load by including only those rivers from southeast Asia and the East Indies that have more than 500 mm y 1 run-off and whose headwaters drain terrain at least 1000 m in elevation. Most of the larger rivers have elevations greater than 3000 m, whereas many of

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Fig. 1. Relationship between annual sediment load (a) and sediment yield (b) and basin area for various southeast Asian and Indonesian=Papua New Guinean humid (>500 mm y 1 run-off), mountain (>1000 m headwater elevation) rivers. Note that the East Indies rivers (Fly, Purari, Solo, Citamandy, Cimanuk, Cimuntur, Cilutung, Cijolang, and Agno; solid dots) have loads and yields very near values predicted based solely on southeast Asian river (open circles) algorithms; see text for further discussion. Data from Milliman and Syvitski (1992), somewhat modified by Milliman and Farnsworth (in prep.).

the smaller rivers have maximum elevations between 1000 and 3000 m. As mentioned above, we did not calculate the loads of rivers with headwaters less than 1000 m in elevation because their sediment loads are small compared to mountain rivers. The data base for southeast Asian river is bimodal with respect to drainage area: rivers with drainage basins generally larger than 200 000 km2 (Changjiang, Ganges, Bramaputra, Mekong, Zhujiang, Irrawaddy and Hong rivers) compared with 15 small Taiwanese rivers whose drainage basins are mostly less than 3000 km2 in area. We also have included data for the nine East Indies rivers for which we have reliable data — the Purari and Fly (Papua New Guinea), the Cimanuk, Citamandy, Cimuntur, Cilutung, Cijolang, and Solo (Indonesia), and the Agno (Philippines) (Milliman and Farnsworth, in prep.). The basin area–sediment load trend for the nine East Indies rivers (y D 3:5x 0:76 ; r 2 D 0:77) is reasonably close to the trend for the southeast Asian rivers (y D 5:3x 0:62 ; r 2 D 0:80). We therefore used the area–load algorithm for the combined southeast Asian–East Indies data set, which is essentially the same as for the southeast Asian data alone (Fig. 1).

Local conditions, of course, can result in higher or lower loads (and yields) for specific rivers. Because we have calculated the loads for more than 600 rivers, however, we assume that rivers with over-estimated loads are balanced out by rivers whose loads have been underestimated. Rivers draining heavily forested areas, for example, should have significantly lower loads than rivers draining similar sized basins that have been extensively logged (e.g. Douglas, 1996). Moreover, it must be emphasized that not all of the sediment necessarily reaches the ocean. Some may be deposited in flood plains of the lower delta, in estuaries or in mangroves. Harris et al. (1993), for instance, calculated that about 30% of the Fly River sediment load is deposited along the delta front (accounting for shoreline progradation), and another 30% accumulates on the pro delta.

3. Flux of fluvial sediment The islands of New Guinea, Borneo and Sumatera are among the largest islands in the world, with a combined area of 1:9ð106 km2 ; total land area of the

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Table 1 Calculated collective sediment loads and yields of mountain and high-mountain rivers draining six high-standing islands Island

No. rivers

Cum. area (ð103 km2 )

Percent Island area

Calculated sed. load (109 t y 1 )

Av. yield (t km2 y 1 )

Sumatra Java Borneo Celebes Timor New Guinea Total sediment load

82 37 36 42 15 253 a

330 98 570 94 16 745

78 78 80 53 45 93 a

0.78 0.33 0.91 0.35 0.1 1.7 a 4:2 ð 109 t y

2400 3400 1600 3700 6200 2300 a 1

a Data for New Guinea include upland (500–1000 m maximum elevation) rivers although quantitatively these rivers discharge relatively little sediment.

six islands is 2:3ð106 km2 . Total drainage basin area of the rivers analysed in this study is 1:8 ð 106 km2 , representing about 80% of the combined drainage area of these islands. The remaining 20% mostly consists of river basins draining lower elevations; interior drainage is assumed to be minor. The greatest sediment loads are calculated to be discharged from Papua New Guinea, Borneo and Sumatera (1.7, 0.9 and 0:8 ð 109 t y 1 , respectively), while much lower loads are calculated for the smaller islands of Jawa, Sulawesi and Timor (0.3, 0.4 and 0:1ð109 t y 1 , respectively). Because smaller islands are dominated by small to very small rivers, however, the sediment yields for Jawa, Sulawesi and Timor are 2 to 3 times greater than for the three large islands (Table 1). Total sediment discharge for the six islands is estimated to be about 4:2 ð 109 t y 1 , which represents about 20 to 25% of the total estimated sediment discharge to the global ocean (Milliman and Syvitski, 1992; Ludwig and Probst, 1996), even though these six islands only account for about 2% of the land area draining into the global ocean.

4. Fate of fluvial sediment The arrows depicted in Fig. 2 indicate that the sediment leaving the six islands can experience several different fates. Some of the sediment, most notably that discharged from the eastern and northern sides of Sumatera and Java, the southern side of New Guinea, and much of Borneo, discharges onto broad shallow shelves. We assume that much of this sediment is retained on the shelves and that little

may escape to deeper water. In contrast, the shelves west and south of Sumatera and Java, much of Sulawesi and Timor, and the north coast of New Guinea are narrow, locally less than 1 km wide. Moreover, because many rivers lack estuaries, they discharge directly into the ocean. As such, sediments may by-pass the shelf and be transported almost directly onto the slope and deeper water beyond, particularly during floods. In the following paragraphs we discuss several seismic profiles taken in June 1997 off Papua New Guinea aboard the Australian research vessel Sir John Franklin. This discussion is followed by some published oceanographic data that suggest that at least some of the fluvial sediment may escape the coastal waters and be transported large distances out to sea. 4.1. Seismic profiles High-resolution seismic profiles were collected off the Sepik River on the north coast of New Guinea and in the Gulf of Papua northwest of Port Moresby (Fig. 3). The seismic system was a 450-joule 100electrode sparker. Shots were fired every 4 seconds (or, assuming an average ship’s speed of 4.5 knots, about every 9 m along the track) and were tied to ship’s navigation. Data were collected and replayed on an Elics Delph2 digital acquisition system. More than 800 km of seismic data were collected on this cruise. Several selected profiles are shown in this paper, as they provide an idea as to the different fates of the sediment discharged to the narrow, active margin to the north and the broad passive margin to the south.

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Fig. 2. Sediment discharge (106 t y 1 ) from the six East Indies islands. Arrow width is proportional to annual load. Letters S, J, B, C, T. and NG refer to Sumatera, Jawa, Borneo, Sulawesi (Celebes), Timor and New Guinea, respectively. Shaded areas represent water depths less than 1000 m, although most of these areas are less than 100 m in depth.

The northern shelf is generally very narrow, locally less than 1–2 km in width, which considerably constrained our ability to obtain sufficiently long profiles to delineate the shallow seismic character on the shelf. A somewhat broader plateau north of the Ramu River, however, did permit us to obtain both along-shore and on-shore=off-shore profiles (Fig. 3). Interestingly, both profiles (Fig. 4) show a broad, relatively thick (up to 30 m) deposit of acoustically transparent and layered sediment, presumably terrigenous mud, overlying a more acoustically opaque layer that outcrops on the outermost shelf in one profile (profile PNG-9708) and is buried in the other (profile PNG-9701B). Given the abundance of coral reefs in this area, we assume that the opaque shelfedge topographic feature is a former reef, the reef crest presently lying at about 95–100 m water depth. As such, this platform probably was an active barrier reef during the last lowstand of sea level. The depth of its crest, however, indicates that by the early Holocene, the reef had been effectively drowned by

sea-level transgression. We assume that this relict reef remained relatively exposed at the seafloor until the Sepik–Ramu inland sea filled about 3 thousand years ago (Chappel, 1993), after which sediment from this river system escaped directly to the shelf and buried the reef. The shelf west of the Sepik River to the west has a thick cover of terrigenous sediment, with no indication of exposed or buried reefs. At present most sediment discharged by the Sepik–Ramu presumably by-passes the shelf, much of it escaping seaward via the Sepik Canyon (Kineke and Sternberg, 1998; Cresswell and Pender, 1998). The acoustically translucent layer seaward of the barrier reef (Fig. 4, Profile PNG-9701A) may represent modern sediment that has buried and bypassed the reef, accumulating on the outer shelf and upper slope. Seismic profiles from the Gulf of Papua show a different depositional environment from that off the northern coast. The quantity of sediment entering this area is much greater than any other area within

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Fig. 3. Location of seismic profiles taken aboard the RV Sir John Franklin, June 1997. The two profiles shown in Fig. 4 were taken north of the Ramu River off the north coast of Papua New Guinea (a), and those in Fig. 5 were taken in the Gulf of Papua (b). Because of their length, only portions of the profiles are shown in this paper.

New Guinea, and quite possibly greater than the sediment flux from any similarly sized area in the East Indies. A number of large rivers discharge here, the most prominent being the Fly, Purari and Kikori. Collectively, the rivers draining into the Gulf are estimated to discharge approximately 400 km3 of water annually (Wolanski et al., 1984) and 300 ð 106

t of sediment (Milliman, 1995), the Fly and Purari together accounting for about half of this total. The mid-shelf of the Gulf is covered with a clinoform deposit of modern mud, locally more than 30 to 40 m thick (Fig. 5). Accumulation rates along the inner shelf can exceed 1.5 cm y 1 (Walsh and Nittrouer, 1998). (Unfortunately, uncharted waters

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Fig. 4. E–W (Profile PNG-9701B) and S–N (Profile PNG-9708) high-resolution seismic profiles off northern Papua New Guinea, north of the Ramu River. This narrow shelf may have been a barrier reef (arrows) and lagoon system during the last low stand=transgression of sea level. The relict lagoon presumably was filled after sediment from the Sepik–Ramu filled the low-lying terrain landward of the present-day coast and began prograding over the shelf, about 3 ka according to Chappel (1993). Note that mud (presumably from the Ramu River) has buried the reef to the east, escaping to the slope and beyond. Depths in meters are calculated assuming a velocity in both water and sediment of 1500 m s 1 .

prevented us from profiling in the inner shelf in the Gulf.) Locally the clinoform muds contain biogenic gas, rendering the seismic profiles acoustically opaque (Fig. 5, Profile PNG-9728). The clinoform thins seaward to less than 1–5 m on most of the outer shelf (see western edge of Profile PNG-9719). In some areas the outer shelf is characterized by surface or buried megaripples (e.g., Profile PNG-9728), suggesting this to be a high-energy environment and that considerable sediment may bypass this area (Harris et al., 1993). Even off the Fly River, trans-

gressive sediments and older Pleistocene sediments are often covered by only a thin cover of modern sediment (Harris et al., 1996). Carbonate sands are common on the outer shelf, and active coral reefs and Halimeda bioherms grow on the outer shelf off the Fly River (Harris et al., 1996), further confirming the lack of modern fluvial sediment accumulation beyond the inner and mid-shelf. Our high-resolution seismic profiles show what may be relatively thick modern deposits in the upper reaches of the Papua Trough (profiles not shown in

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Fig. 5. N–S (PNG-9728) and E–W (PNG-9719) high-resolution seismic profiles in the Gulf of Papua. As in Fig. 4, only parts of each profile are shown. Distance between the two parts of profiles in PNG-9728 is approximately 1.5 km, and for PNG-9719 it is approximately 18 km. Note the presence of biogenic (?) gas at the beginning of PNG-9728 and the buried mega-ripples further offshore. In profile PNG-9719, the inner shelf mud wedge is about 40 m thick, and this wedges out at the western edge of the profile. Arrows in both profiles refer to the inferred transgression-Holocene surface. Depths in meters are calculated assuming a velocity in both water and sediment of 1500 m s 1 .

this paper), but Pb-210 data from cores taken in the Trough indicate accumulation rates of 1 mm y 1 (Walsh and Nittrouer, pers. comm.). This suggests that relatively little sediment escapes the inner shelf — or at least the little that may escape remains in the upper parts of the Trough. A somewhat analogous situation can be seen off the Mahakam River in southeastern Sulawesi, where fluvial sediment accumulates near the coast, but large amounts apparently are swept westward by local currents (Roberts and Sydow, 1996). The lack of offshore transport is evidenced by the abundance of Halimeda bioherms that dominate foreset beds within 20 km of the delta front (e.g., Roberts and Sydow, 1996, their Figs. 2–4).

4.2. Oceanic links Most marine scientists currently believe that fluvial sediment tends to be deposited and remain relatively near to its discharge point. Some sediment may be moved offshore, and perhaps even be transported laterally some distance, but the regional nature of clay mineral distribution in the oceans (e.g., Biscaye, 1965; Griffin et al., 1968) offers strong evidence for regional accumulation of fluvial sediment, as do the presence of deltas and submarine fans off most large and medium-size rivers. The scattered seismic data off Papua New Guinea, as well as both published and unpublished data from other researchers, indicate that much if not most of the sediment trans-

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ported to broad passive margin shelves remains on the inner to middle portions of the shelves (e.g., Meade, 1996). In contrast, sediment discharged on to narrow shelves is more likely to by-pass to the slope and beyond. Filled trenches (Hamilton, 1979; cf., Milliman, 1995) suggest that off northern New Guinea much of this by-passed sediment is deposited downslope of its point of entry. More recent oceanographic data, however, offer the intriguing possibility that some of the sediment discharged from the East Indies rivers may be transported far greater distances than previously supposed. As part of the Joint Global Ocean Flux Study (JGOFS) multi-discipline research in the eastern equatorial Pacific (EqPac) Coale et al. (1996) and Gordon et al. (1997) have reported anomalous concentrations of particulate aluminum in 150– 200 m-depth waters straddling the equator at 140ºW (Fig. 6). The particulate aluminum is associated with reactive iron, whose availability is considered a major limitation to phytoplankton production in the open ocean (Martin, 1992). This iron, in fact, may be the ‘fertilizer’ responsible for much of the productivity associated with equatorial upwelling in the eastern Pacific (e.g., Fitzwater et al., 1996).

Fig. 6. S–N vertical section equatorial Pacific (FeLine 1990 cruise) at 140ºW showing the particulate aluminum anomaly in the upper water column at the equator, coincident with the core of the Equatorial Counter Current (after Gordon et al., 1997). One possible explanation for this anomaly is that it represents river-derived alumino-silicates transported from the west, although Gordon et al. (1997) have concluded that the aluminum anomaly probably was derived from hydrothermal vents activity.

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Gordon et al. (1997) reason that because the particulate aluminum anomaly occurs within the Equatorial Counter Current (ECC), it must originate from the west. The aluminum might represent terrigenous alumino-silicates, which “ : : : could originate : : : in and around the coastal areas of Papua New Guinea : : : ” (Gordon et al., 1997, p. 421). Co-existing elevated levels of particulate manganese along the 140ºW transect, however, have led Gordon and coauthors to conclude that the manganese, iron and aluminum are precipitates derived from shallow hydrothermal vents near Papua New Guinea, although the exact location of these vents is not clear. On the other hand, we find several lines of evidence that argue against a hydrothermal origin. First, hydrothermal events tend to be episodic, so that elevated particulate aluminum and iron within the ECC also should be sporadic in their distribution if derived from hydrothermal activity. But other findings of similarly elevated particulate concentrations in the ECC (see discussion in Gordon et al., 1997) suggest that this feature may be more or less permanent, which would indicate a more constant input, such as from fluvial runoff. Second, although few hydrothermal plumes have been sampled geochemically, Feely et al. (in press) show that the particulate Al=Fe and Mn=Fe ratios for plumes emanating from the Gorda Ridge (northeastern Pacific) range from 0.026 to 0.39, and 0.0021 to 0.01, respectively; higher ratios from chronic plumes on and near the Gorda Rise (1.35 and 0.058, respectively) indicate to Feely et al. a contribution of resuspended sediment. That the particulate Al=Fe and Mn=Fe ratios reported by Gordon et al. (1997) within the ECC are about 1.5 and 0.25, respectively, would thus seem to reflect a non-hydrothermal origin. Because the data are still too fragmentary to offer conclusive evidence, we prefer a fluvial origin for the elevated aluminum concentrations found in the eastern tropical Pacific, and derivation from the northern coast of Papua New Guinea (as suggested by Gordon et al., 1997) seems the most likely source. Assuming a constant concentration of particulate aluminum (0.4 nmol=kg) throughout the core of the ECC (6500 by 200 by 0.2 km) and a mean current speed of 50 to 100 cm s 1 (Halpen et al., 1988; G. Cresswell, 1998, oral communication), the quantity of suspended alumino-silcates transported by ECC would range be-

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tween 104 and 105 t y 1 , which represents much less than 0.1% of the 109 t y 1 discharged eastward from the East Indies rivers. The problem, of course, is to explain how this fine-grained sediment could remain suspended in the upper portion of the water column throughout the 6500-km transit from its fluvial source(s) in the western Pacific. Utilizing the published data of Levitus (1982), Richard Dugdale (pers. comm., 1998) also has noted a surface plume of dissolved silica flowing west from the East Indies. While it is tempting to suppose that this silica anomaly also may represent fluvial input, at present there are no data to confirm this possibility. Thus there are several lines of evidence, although too equivocal to allow any firm conclusions, that infer that the influence from East Indies rivers may extend a great distance the rivers’ mouths. Suspended particulates, however, do not seem to have been sampled at depth in the western Pacific and eastern Indian Ocean (J.W. Murray, 1998, pers. comm.). Thus, while the data may suggest long-distance transport of fluvial sediment in the equatorial Pacific and (perhaps) Indian oceans, such a hypothesis cannot be confirmed without acquiring large-volume water samples.

physical data from the margins and deep-sea adjacent to these high standing islands. Swath bathymetry, which is notably lacking in this region, can give us a picture of the topographic constraints and effects of high sediment flux, and high-resolution seismic profiles can provide the data necessary to delineate areas of both sediment deposition and erosion. Of course, these geophysical data must be groundtruthed, which will require extensive coring, as well as a variety of age-dating to validate our preliminary observations about rates of sediment accumulation. (3) Finally, while the data are few and by no means totally convincing, the possibility of some fluvial sediment presently escaping both east and west of the East Indies is especially intriguing. Of particular importance would be large-volume sampling of the upper water column in equatorial western Pacific and the eastern Indian Ocean. Not only could these data provide a possible, previously unrecognized source of nutrients and fertilizer to the equatorial ocean, but they also could support the suggestion that fluvial–ocean interactions may extend further from the land–ocean interface than previously envisioned.

Acknowledgements 5. Concluding remarks The large quantity of sediment calculated to be discharged by six large oceanic islands in the East Indies lends credence to Milliman’s and Syvitski’s (1992) suggestion that the East Indies (Oceania) is the single largest regional source of sediment to the global ocean. Based on the results presented in this paper, there are a number of directions that future research should follow: (1) More data on East Indies fluvial discharge should be obtained, preferably through actual field study; but if this is not possible, by additional calculations using new and improved algorithms. These data are particularly important in view of the increased deforestation on many East Indies islands, which may well increase the sediment yields of these small rivers manifold (Douglas, 1996) in the coming years. (2) If we are to understand the fate of these sediments, we need additional sedimentological and geo-

We were honored to have been asked to write this paper in recognition of Doeke Eisma’s retirement from the Netherlands Institute for Sea Research. Doeke has been a friend and colleague of the senior author for many years, and we hope that he will continue to interact with both the Dutch and international marine community, as we already miss his keen insights and intriguing ideas as well as his good companionship. We thank our Australian colleagues (particularly Ken Woolfe and Gregg Brunskill) and the officers and crew of the Australian research ship, RV Sir John Franklin for allowing us to sail with them in Papua New Guinean waters. Gail Kineke and J.P. Walsh contributed to parts of Fig. 3. Discussions with Chuck Nittrouer, J.P. Walsh, Dick Dugdale and Jim Murray also helped us focus our thoughts, for which we are grateful. Tjeerd Van Weering, Nick McCave and Johan Van Bennekom offered valuable suggestions to a draft manuscript. This research

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was sponsored in part by the US National Science Foundation (OCE 9222405) and the Office of Naval Research (N00014-94-0179). This is contribution no. 2185 from the Virginia Institute of Marine Science. References Biscaye, P.E., 1965. Mineralogy and sedimentation of recent deep-sea clay in the Atlantic Ocean and adjacent seas and oceans. Geol. Soc. Am. Bull. 76, 803–822. Chappel, J., 1993. Contrasting Holocene sedimentary geologies of the lower Daly River, northern Australia, and lower Sepik– Ramu, Papua New Guinea. Sediment. Geol. 83, 339–358. Coale, K.H., Fitzwater, S.E., Gordon, R.M., Johnson, K.M., Barber, R.T., 1996. Control of community growth and export production by upwelled iron in the equatorial Pacific Ocean. Nature 379, 621–624. Cresswell, G.R., Pender, L.F., 1998. The Sepik River outflow into the Bismark Sea, May 1997 (abs.), AGU-ASLO Ocean Sciences Meeting, San Diego, OS 14. Douglas, I., 1996. The impact of land-use changes, especially logging, shifting cultivation, mining and urbanization on sediment yields in tropical Southeast Asia: a review with special reference to Borneo. Int. Assoc. Hydrol. Sci. Publ 236, 463– 472. Feely, R.A., Baker, E.T., Lebon, G.T., Gendron, J.F., Massoth, G.J., Mordy, C.W., in press. Chemical variations of hydrothermal particles in the 1996 Gorda Ridge event and chronic plumes. Deep-Sea Res. II. Fitzwater, S.E., Coale, K.H., Gordon, R.M., Johnson, K.S., Ondrusek, M.E., 1996. Iron deficiency and phytoplankton growth in the equatorial Pacific. Deep-Sea Res. II 43, 995–1015. Gordon, R.M., Coale, K.H., Johnson, K.S., 1997. Iron distributions in the equatorial Pacific. Implications for new production. Limnol. Oceanogr. 42, 419–431. Griffin, J.J., Windom, H., Goldberg, E.D., 1968. The distribution of clay minerals in the world ocean. Deep-Sea Res. 15, 433– 459. Halpen, D.R., Knox, R.A., Luther, D.S., 1988. Observations of 20-day period meridional current oscillations in the upper ocean along the Pacific equator. J. Phys. Oceanogr. 18, 1514– 1534. Hamilton, W., 1979. Tectonics of the Indonesian region. US Geol. Survey. Prof. Pap. 1078, 345 pp. Harris, P., Baker, E.K., Cole, A.R., Short, S.A., 1993. A preliminary study of sedimentation in the tidally dominated Fly River Delta, Gulf of Papua. Cont. Shelf. Res. 13, 441–472.

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Harris, P.T., Pattriaratchi, C.B., Keene, J.B., Dalrymple, R.W., Gardner, J.V., Baker, E.K., Cole, A.R., Mitchell, D., Gibbs, P., Schroeder, W.W., 1996. Late Quaternary deltaic and carbonate sedimentation in the Gulf of Papua foreland basin: response to sea-level change. J. Sediment. Res. 66, 801–819. Kineke, G.C., Sternberg, R.W., 1998. Suspended sediment dispersal from the Sepik River, Papua New Guinea (abs). AGUASLO Ocean Sciences Meeting, San Diego, OS 14. Levitus, S., 1982. Climatological atlas of the world ocean. NOAA Prof. Pap. 13. US Government Printing Office, Washington, DC, 173 pp. Ludwig, W., Probst, J.-L., 1996. A global modeling of climatic, morphologic, and lithological control of river sediment discharges to the ocean. Int. Assoc. Hydrol. Sci. Publ. 236, 21– 28. Martin, J.H., 1992. Iron as a limiting factor in oceanic productivity. In: Falkowski, P., Woodhead, A.D. (Eds.), Primary Productivity and Biogeochemical Cycles in the Sea. Plenum Press, New York, pp. 123–127. Meade, R.H., 1996. River-sediment inputs to major deltas. In: Milliman, J.D., Haq, B.U. (Eds.), Sea-Level Rise and Coastal Subsidence. Causes, Consequences, and Strategies. Kluwer, Dordrecht, pp. 63–85. Milliman, J.D., 1995. Sediment discharge to the ocean from small mountainous rivers: the New Guinea example. Geo-Mar. Lett. 15, 127–133. Milliman, J.D., Meade, R.H., 1983. World-wide delivery of river sediment to the oceans. J. Geol. 91, 1–21. Milliman, J.D., Syvitski, J.P.M., 1992. Geomorphic=tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. J. Geol. 100, 525–544. Milliman, J.D., Ren, M.-E., Qin, Y.-s., Saito, Y., 1987. Man’s influence on the erosion and transport of sediment by Asian rivers: the Yellow River (Huanghe) example. J. Geol. 95, 751– 762. Pinet, P., Souriau, M., 1988. Continental erosion and large-scale relief. Tectonics 7, 563–582. Roberts, H.H., Sydow, J., 1996. The offshore Mahakam Delta: Stratigraphic response of late Pleistocene-to-modern sea level cycle. Proc. Indonesian Petrol Assoc., IPA96-1.1-031, pp. 147–161. Walsh, J.P., Nittrouer, C.A., 1998. Sedimentation processes in the Gulf of Papua and the Fly River mouth, Papua New Guinea (abs.). AGU-ASLO Ocean Sciences Meeting, San Diego, OS 32. Wolanski, E., Pickard, G.L., Jupp, D.L.P., 1984. River plumes, coral reefs and mixing in the Gulf of Papua and the northern Great Barrier Reef. Estuar. Coast. Shelf Sci. 18, 291–314.