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doi:10.2204/iodp.proc.320321.210.2013

Results

Concentrations of dissolved nitrate and silicate for all eight Expedition 320/321 sites are provided in Tables T3, T4, T5, T6, T7, T8, T9, and T10 and Figures F1 and F2. Depths are plotted in core depth below seafloor, method A (meters below seafloor [mbsf] in text and figures), for all sites.

The profile of dissolved nitrate concentrations from our measurements on whole-round and Rhizon samples at Site U1331 shows a slight decrease with depth, starting at ~70 µM at 2.95 mbsf and decreasing to an average of ~55 µM at depths greater than 22 mbsf. At Site U1332, concentrations remain between ~55 and 45 µM. Nitrate concentrations at Site U1333 range from 50 to 60 µM and increase to 83 µM at 35.95 mbsf. Site U1334 contains the highest nitrate measured at any site, with concentrations in the shallowest sample at ~261 µM followed by a gradual decline from ~44 µM at 6 mbsf to ~10 µM at 125 mbsf and a slight increase to ~50 µM at 228 mbsf. Site U1335 follows a similar profile, with high nitrate in the shallowest samples (~153 µM), a sharp decline to near-zero values (54–375 mbsf), and a slight increase at depth (~34 µM). Near-surface concentrations at Site U1336 start at ~75 µM and steadily decrease to ~10 µM by 93 mbsf. At Site U1337, nitrate remains below 30 µM from 7 to 409 mbsf and increases to ~64 µM at ~428 mbsf. Site U1338 also remains low (<75 µM) from 1 to 350 mbsf and increases at 360 mbsf to 200 µM. Deepwater nitrate concentrations from the World Ocean Circulation Experiment Pacific Ocean Atlas in the equatorial Pacific (Line P04; 10°N, –120°W; ~4500 m water depth) proximal to Expedition 320/321 sites are steady at ~36 µM (Talley, 2007).

Profiles of dissolved silicate concentrations from our measurements on whole-round and Rhizon samples at most sites increase with depth. At Site U1331, surface concentrations of ~750 µM at 3 mbsf approach 900 µM at 123 mbsf. Surface concentrations at Site U1332 contain the lowest measured silicate concentrations of ~555 µM at 3 mbsf and approach 1200 µM at 103 mbsf. Profiles of silica at Sites U1333, U1335, and U1336 remain close to ~1000 µM. Silicate at Site U1334 increases from ~900 µM at shallow depths to 1362 µM at 257 mbsf. Silicate at both Sites U1337 and U1338 increases from ~1000 µM to the highest measured values of more than ~1500 µM. Rhizon sampling at Site U1338 also targeted a diatom-rich interval at ~130 mbsf (Section 321-U1333B-14H-5; see the “Site U1338” chapter [Expedition 320/321 Scientists, 2010e]) and contains average silicate concentrations of ~1150 µM.

Silicate concentrations measured in this study (including Rhizon samples) are consistently higher than shipboard measurements from Expedition 320/321 (0.67 µM/µM; R2 = 0.41; n = 380), falling below a 1:1 line (Fig. F3). When compared by expedition, our data are on average more similar to shipboard measurements made during Expedition 321 (Sites U1337 and U1338; 0.56 µM/µM; R2 = 0.40; n = 153) than during Expedition 320 (Sites U1331–U1336; 0.19 µM/µM; R2 = 0.07; n = 227).

Rhizon samples from a 1.3 m interval at Site U1331 (Section 320-U1331B-1H-6; Table T3) were collected to test the feasibility of using the Rhizon sampling procedure during Expedition 320/321. Shipboard measurements and our archived samples show general agreement between whole-round and Rhizon sample concentrations (Table T3; Figs. F1, F2) (Expedition 320/321 Scientists, 2010b).

Differences in concentrations between whole-round and Rhizon samples at Sites U1331 and U1334 are partially attributed to samples originating from different holes (i.e., whole-round samples at Site U1331 are from Hole U1331A and Rhizon samples are from Hole U1331B; Table T3). Additionally, interstitial water sampling from extended core barrel–cored sections may contain contamination from seawater drilling fluid (i.e., Samples 320-U1334C-23X-1, 75 cm, to 23X-3, 75 cm, as well as Site U1337 below 210 mbsf [Section 321-U1337A-23X-3] and Site U1338 below 244 mbsf [Section 321-U1338A-27X-3) (Expedition 320/321 Scientists, 2010c, 2010d, 2010e).