Vertical flux, ecology and dissolution of radiolaria in tropical oceans : implications for the silica cycle
Radiolarians which settle through the oceanic water column were recovered from three stations (western Tropical Atlantic-Station E, central Tropical Pacific-Pi and Panama Basin-PB) using PARLUX sediment traps in moored arrays at several depths. The taxonomic diversities of the radiolarian assemblages in the sediment traps were very high. A total of 420 taxa, including 23 newly identified taxa, were found at the three stations; of these, 208 taxa were found at station E. The polycystine radiolarians generally reach the sea floor with little change in abundance or species composition, although slight skeletal dissolution occurs throughout their descent. The phaeodarian radiolarians, on the other hand, are largely dissolved within the water column; only a few species reach the sea-floor and these dissolve rapidly at the sediment-water interface. Most radiolarian skeletons sink as individuals through deep water columns without being incorporated into large biogenic aggregates. Because significant numbers of nassellarian and phaeodarian species are deep-water dwelling forms the diversity index of radiolarians increases with increasing depth in the mesopelagic zone. The vertical flux of the total radiolarians arriving at the trap depths (in x 103 individuals/m2/day) ranged from 16-24 (E), 0.6-17 (Pl), and 29-53 (PB). Of these on the average 25% and 69% of the total radiolarian flux is transported by Spumellaria and Nassellaria, respectively, while 5% is carried by Phaeodaria. The measured Si02 content of the skeletons averaged 91, 98 and 71% of measured weight for Spumellaria, Nassellaria and Phaeodaria, respectively. The supply of radiolarian silica (mg Si02/m2/day) to each trap depth ranged from 2.5-4.0 (E), 0.9-3.2 (Pl), and 5.7-10.4 (PB). The Radiolaria appear to be a significantly large portion of the Si02 flux in >63 pm size fraction and thus play an important role in the silica cycle. When the radiolarian fluxes at the three Stations are compared with Holocene radiolarian accumulation rates in the same areas it became apparent that several percent or less of the fluxes are preserved in the sediments in all cases and the rest is dissolved on the sea-floor. Estimated excess Si which is derived from Si02 dissolution on the sea-floor is fairly small relative to advective Si in the western North Atlantic and thus it appears to be insignificant to show any deviation in a simple mixing curve of deep water masses. Weight, length, width, projected area and volume of 58 radiolarian taxa were measured. The density contrast of radiolarians, relative to seawater, generally falls between 0.01 and 0.5 g/cm3. The sinking speed of 55 radiolarian taxa, measured in the laboratory at 3°C, ranged from 13 to 416 m/day. Despite the wide variety of morphology between the species, sinking speeds were best correlated with weight/shell among all the possible combinations of the examined variables. The estimated residence times of these taxa in the 5 km pelagic water column ranged from 2 weeks to 14 months. Large phaeodarians reached the water-sediment interface relatively quickly and ultimately dissolved on the sea floor. Small-sized taxa dissolved en route during sinking. The standing stock of 26 examined abundant taxa is on the order of 1 to 100 shells/m3. Total radiolarian standing stock ranges from about 450 shells/m3 at Stations Pl and E to 1200 shells/m3 at Station PB. The rate of production of total Radiolaria is calculated to be 77 to 225 shells/m3 /day. The turnover time for these species ranges from several days to one month depending on the species and the assumption of the depth interval used for the estimation.
Radiolaria, Fossil, Marine sediments, Atlantis II (Ship : 1963-) Cruise AII108-2
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Massachusetts Institute of Technology and Woods Hole Oceanographic Institution