Dataset: MULVFS Particulates K2

As part of the VERTIGO project, Multiple Unit Large Volume in-situ Filtration System (MULVFS) sampling took place during two week-long intensive study periods at ALOHA (in 2004) and K2 (in 2005). This dataset includes data from K2.

MULVFS was first described in Bishop et al., 1985. MULVFS consists of 12 ship-electricity powered pump units deployed simultaneously to kilometer depths using a dedicated (unified) 1000 m long electromechanical cable and winch system. MULVFS sample depths were 10, 35, 60, 85, 135, 185, 235, 310, 460, 560, 660, 760, and 810 m at K2. The shallowest sample was always within the surface mixed layer. VERTIGO casts were timed to capture particles near local noon and midnight to investigate the effects of diurnal zooplankton migrations on particle distributions. At K2, 2 day/night pairs were obtained.

Each pump unit can collect samples of particulate and dissolved species using three flow paths. Check and gas release (de bubbler) valves protect filter samples from the effects of back flow, contamination, and disruption due to trapped air on deployment and degassed air expansion on recovery. The later is a problem in shallow samples.

Depending on depth and particle concentration, 2000–16 000 L volumes of seawater are filtered under a suction of ~0.8 atmospheres over 4–5 h through the main multi-stage (3 anti-washout baffles and two filter stages) filter holder. The first anti-washout baffle is a heavy polyethylene plastic cover with incised 1 cm scale triangular flaps centered over each of the 52 tubes of the second baffle stage and was added to ensure particle retention under strong current shears.

The main filter series (with an effective filtration diameter of 24.5 cm) consists of a 51 um polyester weave mesh prefilter supported by 149 um polyester mesh and 1.2 cm spaced 1.2 cm thick plastic grid in the prefilter stage, followed by two identical Whatman QMA quartz fiber filters supported by 149 um polyester mesh and 149 um porous polyethylene frit. All filters and components are acid cleaned. The three particle size fractions represented by prefilter and QMA filters are >51, 1–51, and <1 um. Fiber flters are 'depth' filters and particles are captured from the flow by the fibers, not pores. Thus a second filter captures additional small particles that pass through the first (Bishop and Edmond, 1976). The "<1 um" fraction, thus represents some of particles in the larger submicron particle class (Bishop et al., 1977, 1985).

The second flow path, with 500–2000 L water flow capacity was used for separate multi-stage filter assemblies and in-line Mn radionuclide adsorption cartridges (Charette et al., 1999). Refer to the Supplemental Document "VERTIGO MULVFS Methods" for the filter assembly used by T. Trull (University of Tasmania, UTAS) The third ('side arm') flow path was used for simultaneous attachment of up to six 47 mm filter holders or smaller absorbers. We used two for separate quantification of >0.4 um Si and for >0.4 um Ba and Mn (Poretics Polycarbonate, 0.4 um, Osmonics, Inc.). About 30% of the time all side arm filter holders had a common 0.4 um filter and the volume was apportioned by number of samples collected. When different filters/adsorbers were used we estimated flow through each type apportioned by flows measured under suction aboard ship.

Blank samples (filters mounted on a non-operating pump and lowered to depth for the cast duration) were processed identically to all other samples.

Aboard ship, MULVFS samples were photographed under controlled lighting using a NIKON COOLPIX 5700 digital camera (Lam and Bishop, 2007) and processed in depth order within 2 hours of the end of cast. All work was performed in a class 100 laminar- flow bench; non-contaminating gloves, sub sampling templates, scalpels, and tweezers were used. QMA filters were sub sampled for up to six other investigators using sharpened acid leached ~45 mm diameter acrylic tubes; "pie slice" sub samples equivalent to 1/8 to 1/4 of each >51 um sample were cut using a stainless steel scalpel. When rare, larger zooplankton and small fish were removed from the MULVFS prefilter samples prior to sub sampling. The remaining sample was lightly "misted" with 15 mL of 18.2 MOhm Milli-Q water under weak suction (<10 cm Hg) to reduce salt loading, oven dried at 60°C for 1-2 days, and then stored flat in trace metal certified polyethylene bags.

The "Side Arm" 0.4 um filter samples were misted under mild vacuum with ~1.5 mL of Milli-Q water, and then transferred directly to acid leached 125 mL polyethylene bottles in which they were later analyzed. This eliminates fractionation of major sea salt components (e.g., Na from Ca).

Sample processing at Lawrence Berkeley National Laboratory (LBNL) was performed in a class-100 laminar flow fume hood in a class-10000 laboratory environment and followed Bishop et al., (1977, 1985). For analysis by Inductively Coupled Plasma Mass Spectrometer (ICP-MS), ~1/50th of each QMA filter was sub sampled following Bishop et al. (1985) and ~1/40th of each 51 um prefilter was cut using a stainless steel scalpel (guided by eye to avoid zooplankton) using a rectangular acrylic template with recessed center. Sub samples were transferred to separate 125 mL acid leached - dried Nalgene® polyethylene bottles, flooded with 10 mL of 0.6 N ultra pure HCl (Seastar Chemicals Baseline Acid), and heated at 60°C overnight (~16 hours). Each leach solution was filtered (0.4 um) and then diluted with 18.2 MOhm Milli-Q water to 50 grams weight. Solutions were further diluted (1:4) with Milli-Q water and treated with an indium spike (final In conc ~0.7 ppb) and then analyzed using a Finnigan Element II ICP-MS. Mixed element standards, and CASS III seawater, were prepared at multiple dilutions in the same 0.12 N HCl matrix. Elements determined were Li*, Na, Mg*, Al, P, K*, Ca*, Cr, Mn, Fe, Co, Ni, Cu, Zn, Rb, Sr*, Y, Cd, I, Cs, Ba, Tl, Pb, Bi, Ce, Nd, U*. Elements marked with asterisks (*) were corrected for sea salt components using Na (Bishop et al., 1977).

Groups from Woods Hole Oceanographic Institution (WHOI) and University of Tasmania (UTAS) were provided fresh filter sub samples for 234Th, C and N, and for Corg, N, δ13Corg, and δ15N, respectively. The >51 um material was rinsed from the polyester "pie slices" using 0.8 um filtered seawater onto 25 mm diameter 0.4 um silver filters, dried at 50 °C, and analyzed as described by Trull et al. (2008). A drying oven mishap resulted in loss of many of the QMA Corg samples from Cast M03 at ALOHA. The 234Th samples were air-dried after/during counting aboard ship and stored frozen in Petri dishes for later analysis for C and N at WHOI. QMA in-situ sample blanks (~3 mg C per 25.4 cm diameter filter) were applied to the data. The use of in-situ blanks compensates for adsorption of dissolved species onto filters during filtration (Turnewitsch et al., 2007).

WHOI Corg and N data for QMA samples from cast M07 and M08 were low by factors of two to three compared with UTAS results. The M08 sub samples analyzed at WHOI had sat wet for considerable time before CN analysis (John Andrews, notes) and the same problem must have impacted their M07 sub samples. Similar problems may have affected some ALOHA samples from Cast M04, which also were not oven dried. We report all data but use the WHOI Corg QMA data for ALOHA (M1, M2, M4, M5) and the UTAS (combined <1 and 1-51 um results) QMA Corg and N data for K2; The <1 um WHOI results were retained for casts M07, 09, and 10 for comparison purposes. All >51 um Corg samples at K2 and ALOHA were UTAS processed.

Notes from dataset file header:
# Bishop CN calcs as of April 29 2007 results nearly final.
# QMA blanks 25.4 cm diameter filter - all dipped blanks (averaged except for cast 8).
# Trull Filter data as calculated by Buesseler.
# Nitrogen results: many dipped MULVFS sample blanks below detection. The results shown here are an upper limit of N.

# Note nearest surface samples in >51 um size fraction from casts 8 and 10 at night are contaminated with significant numbers of copepods (see Bishop and Wood (2008)).
# treat cast 7 and 8 lt1 and 1-51 PC and PN with caution.
# gt0.4 Corresponds to analysis of a side-arm filter.
# gt51, 1-51, lt1 designations correspond to filtration order as water passes through a 51 um prefilter and paired QMA filters. A single QMA filter quantitatively captures particles >1.2 um; a pair of QMA filters traps particles to 0.8 um size.
# lt51 - sum of paired QMA filter data.
# samples identified by T_ were samples from a separate filter holder.