Figure 2B shows 8-day 4-km composite swaths from 60°N to 20°S and 157°W to 147°W of PIC retrieved from the MODIS AQUA satellite with GP15 stations placed on the corresponding image based on the time of sampling. Figure 2C shows chlorophyll-a swaths with similar station placement. Both views provide spatiotemporal context for the GP15 transect. PIC sensor PIC011 was deployed surface to bottom during large volume in situ pump casts at 21 of 39 stations during GP15. PIC010 was CTD deployed at all stations beginning at station 6 using the GTC CTD/water sampling system.

PIC011 was CTD deployed ∼90 times during the June 2017 California Current ecosystem–Long-Term Ecological Research (CCE-LTER) project process study, aboard R/V Revelle. The study followed the biological evolutions of a biologically productive filament of upwelled cold, high-salinity westward-flowing water off the coast of California (Bourne et al., 2021; Figure 3). Figure 3 provides a spatial context for the study with satellite retrievals of PIC and POC corresponding to days 168 and 169 which bracket the timing of the 100-km-long CTD/optics Transect T2 across the filament.

The sensor concept has been described by Guay and Bishop (2002) and Bishop (2009). The first profiling sensor was a modified version of an analog WETLabs C-Star 25 cm pathlength transmissometer. A 660-nm laser replaced the LED source, and a cell with high crossing efficiency polarizers (630–700 nm, Polarcor, Corning) was inserted into the water path length of the instrument. At the source end, the polarizer is aligned with the plane of polarization of the laser; on the receiver end, the polarizer is crossed, thus minimizing the detection of the primary beam. The sensor thus detects the photon yield resulting from the interaction of polarized laser light with birefringent particles in the beam. The first full water column profiles of the first sensor (PIC001) took place in 2003 in the North Atlantic (Bishop, 2009). This sensor was stabilized in 2006 by replacing the cell with body-mounted polarizers.

The primary challenge during the development was to maintain the optical fidelity and electronic stability of the instrument as it is being buffeted by turbulence at profiling speeds of 1 m s⁻¹, with pressures ranging from 100 to 60,000 kPa (1–600 atm), temperatures ranging from −1 to 35 C, and thermal shocks as large as 1°C s⁻¹. The polarizing elements of the sensor must be perfectly pressure-compensated yet mounted robustly in a way to minimize stress-induced polarization effects. The latter was a major problem. Over many iterations of the basic design and multiple sea trials, we demonstrated in 2013 that multiple PIC sensors yielded identical results and exceeded the performance of PIC001 (Supplementary Figure S1).

Protocols for pump deployments, sample collection, and analysis are described by Xiang and Lam (2020). Briefly, size-fractionated particles were collected using McLane Research in situ pumps (WTS-LV) that had been modified to accommodate two flow paths. At most GP15 stations, two casts of eight pumps (a shallow cast to 1,000 m and a deep cast from 1,000 m to near bottom) enabled sampling at 16 depths; at super stations (Figure 2A), three casts [shallow (s), midwater (m), and deep (d)] were achieved. Wire-out was used to target nominal depths during deployment. Typically, shallow, midwater, and deep casts lasted 5–7, 6–8 h, and 8–10 h, respectively. The duration of pump sampling was typically 3–5 h during these casts. An internally logging Seabird 19-plus CTD was deployed at the end of the line and used to verify target depths; true sampling depth was determined by linear scaling from measured end-of-wire depth.

Each pump carried two 142-mm diameter filter holders with two separate filter stages, and multiple baffles upstream of the filter stage were designed to ensure even particle distribution and prevent particle loss (Bishop et al., 2012). One filter holder was loaded with a 51-μm Sefar polyester mesh prefilter followed by paired Whatman QMA quartz fiber filters (identical to the scheme of Bishop and Wood (2008)). The other filter holder was also loaded with a 51-μm prefilter but followed by paired 0.8 μm Pall Supor polyethersulfone filters. The 51 μm prefilters and QMA filter pairs were underlain by a 150-μm Sefar polyester mesh support filter (not analyzed) to facilitate filter handling. Suction from the pump during operation causes water to flow through each filter holder and the dedicated flowmeter. The two metered flows were then metered by a third flowmeter as a crosscheck. Parameters reported in this study were from filter subsamples from both flow paths. The volume of water filtered through each sample ranged from several hundred liters in surface waters to 1,000 L in deep waters. All filters and filter holders were acid-leached prior to being used according to methods recommended in the GEOTRACES sampling protocols (GEOTRACES, 2017). QMA filters were pre-combusted at 450°C for 4 h following acid leaching. Each cast also had a full set of “dipped blank” filters deployed. A 0.2-μm Supor prefilter above the filter set excluded any particles that may have been present in seawater in the filter holder headspace.

PIC010 was mounted horizontally on the GEOTRACES Trace Element Sampling Carousel (GTC) and interfaced to its Seabird 911 CTD. Data from this sensor were acquired at 24 Hz and 12 bit precision (1.25 mV resolution). The C-Star transmissometer (CST1035DR) was also interfaced with the CTD. Data from this instrument were corrected for hysteresis and cross-referenced to data from the pump-deployed transmissometer, CST1450DR. From this comparison, clear water reference values were estimated for CST1035, and beam transmission and beam attenuation coefficients were calculated. The GTC CTD system was deployed 66 times with the PIC sensor. Because the sensor was mounted horizontally, data from both down and upcasts were not influenced by trapped bubbles.

The GP15 transect spanned September 24, 2018 through November 22, 2018 (year days 267–326) from 55°N to 20°S, centered on 152°W. Satellite retrievals of sea surface chlorophyll (Hu et al., 2012), PIC (Balch et al., 2005), and POC (Stramski et al., 2007) at 4-km 8-day temporal resolution from MODIS Aqua and Terra and SNPP VIIRS (Visible Infrared Imaging Radiometer Suite) were downloaded in July 2019 and December 2021 from the NASA ocean color archive for latitudes 60°N to 25°S, and for longitudes from 147°W to 157°W (https://oceancolor.gsfc.nasa.gov/l3/); no differences in property retrievals for MODIS Aqua data from the two time periods were detected. No data are available from the improved (Mitchell et al. (2017)) algorithm for the periods of this investigation.

Zonal transect mean and standard deviation were computed for each of the ten 8-day composites. Data were excluded from the zonal trend when the ratio of standard deviation to mean exceeded 2.0. Transect segments corresponding to the time and location of GP15 stations (Figure 2A) were combined to complete a synthetic transect of retrieved properties from 60°N to 20°S.

The June 1 to July 2 2017 CCE-LTER expedition studied a filament of biologically productive upwelled water that was moving offshore in the vicinity of Morro Bay, CA. Our use of daily resolved 4-km satellite data was described by Bourne et al. (2021). In this study, we used Ocean Data View (Schlitzer, 2021) to extract data within 5 km of the transect from SNPP VIIRS, and MODIS Aqua imagery on ordinal days 168 and 169 which bracketed the times of CTD Transect 2 stations 37 (day 169.118) through 47 (day 169.803). The times of satellite over passes were at 21:40 UTC; thus, images corresponded to ordinal days 168.9 and 169.9. As features in the vicinity of Transect 2 stations were moving at a rate of 5–15 km d⁻¹, a synthetic retrieval of properties was achieved using a time-weighted sum of retrieved properties from the two images (Figure 3).

The GEOTRACES GP15 section is notable as it transited waters of the Aleutian shelf, Alaska gyre, subarctic front, north Pacific gyre, the complex current regime of the equatorial Pacific, and into the south Pacific Gyre. It is further notable by the fact that particle concentrations were very low. The hydrographic context for the GP15 meridional section in the upper 500 m is provided by distributions of temperature, salinity, and oxygen (Figure 7). Cool low-salinity waters present in the upper 50–100 m north of 42°N are characteristic of the subarctic north Pacific. The subarctic front is present from 40°N to 32°N. North Pacific gyre influence is seen from 30°N to approximately 17°N. The properties in the equatorial zone from 17°N through 20°S closely follow the descriptions of Wyrtki and Kilonsky (1984). The core of the westward flowing North Equatorial Current (NEC) which completes the circulation of the north Pacific gyre is found at ∼14°N; the front demarking the North Equatorial Counter Current (NECC) is denoted by strong upward displacement of isotherms and oxygen-depleted waters extending from 100 to 350 m at 11°N. This feature was strongly oxygen deficient with concentrations below 5 µM compared to 20 µM concentrations reported by Wyrtki and Kilonsky (1984). The zone from 4°N to the equator is the northern branch of the westward flowing South Equatorial Current (SEC). Centered at the equator and 150 m depth is the easterly flowing Equatorial under current. The strongest flows of the southern branch of the SEC extend to approximately 8°S, and its influence trails off further south. The signature of the south Pacific Gyre is established at 20°S. There is a second lobe of low oxygen water from 200 to 400 m at approximately 2°S where O₂ values are below 20 µM.

GEOTRACES stations 3 through 10 are in subarctic waters; station 12 is in the subarctic front; stations 14, 16, and 18 are in the oligotrophic north Pacific gyre; station 19 is situated in the NEC; and station 21 is at the NEC/NECC front and underlain by strongly oxygen-deficient waters. Station 23 is in the NECC; stations 25 and 27 lie in the SEC; station 29 sampled the equatorial under current; stations 31 and 33 were in the southern branch of the SEC and underlain by low oxygen waters; stations 35 and 37 were in the frontal waters of the south Pacific gyre; and station 39 is in the south Pacific gyre. The low oxygen waters at stations 21 and 31/33 will be relevant to the interpretation of PIC sensor datasets.

Figures 8A,B compare PIC concentrations derived from birefringent photon yield (PIC011) and analytical values determined in McLane pump samples during the same casts, respectively. We note that sensor data are the results of down and upcast profiles each representing a 10–20 min snapshot of the upper 500 m of the water column, while the pump samples represent averages over the 3–5 h of sample filtration at depth. We also note that sensor data are adjusted for crossing blank drift and using empirically determined pressure coefficients derived from empirical fits of sensor data to measure PIC in waters from 2,000 to 4,000 m with the further constraint that the samples must be shallower than 1,000 m above bottom (mab). The features seen in the upper 2,000 m should be a valid reflection of the performance of the sensors. These results are the first comprehensive ocean-scale validation of the birefringence-derived optical proxy.

The sensor data contoured in Figure 8A are averages over 10 m intervals of all profiles at the same station. Figure 8B shows contours of pump data that have been linearly interpolated at 10 m depth intervals between sample depths; concentrations in the surface layer were assumed constant to the surface. In the subarctic north Pacific, near-surface concentrations for sample PIC peaked at 500 nM at station 8; there was a general factor of two to three underestimation of near-surface PIC derived from PIC011. South of the subarctic front, sensor PIC concentrations in surface waters covary well with measured PIC. Relative minima for PIC from both sensor and samples were found at stations 21 and 33 in the low oxygen zones. A subsurface maximum feature near 120 m is present in sensor data at stations 16 through 19 and is not sampled by the pumps due to sample spacing. The profile shape is very similar to observations made at station ALOHA during the VERTIGO program (Figure 1C). A similar subsurface maximum was seen in sensor data between 150 and 225 m at stations 33 to 39. The relative highs in PIC concentration at depths below 200 m at stations 10 and 12 near the subarctic front and at equatorial station 29 were well reproduced by both sensor and samples.

In deeper waters from 500 m to depths of 1,000 m above bottom (mab), the trends of the data were similar in the sensor and pump data. Relatively high PIC values extended to depth at station 12 near the subarctic front and at equatorial station 29. In the depth interval between the bottom and 1,000 mab, there were cases where sensor and sampled PIC diverged, with sensor data increasing to the seafloor, while measured PIC decreased.

We used 10-m averaged CTD datasets to interpolate sensor data to depths sampled during pumping. Figure 9 shows the relationship between sample PIC and birefringent photon yield from the sensor. The data from the surface to the base of the euphotic layer (PPZ depth, 10% of fluorescence maximum, Buesseler et al., 2020) span a range of slopes from 1:50 to ∼1:7. In the subarctic north Pacific, the near-surface sample PIC values are closer to the 1:50 trend. The data from PPZ depth to 2,000 m are consistently correlated, with fitted trend line of 1:15.4 (R ² = 0.677, n = 130); data from PPZ depth to 1,000 mab yield a trendline of 1:14.5 (R ² = 0.774, n = 190). Data from within 1,000 m of the seafloor follow the midwater trend or trend anomalously to high birefringence. These anomalous results are likely due to the effects of resuspended clays.

Figure 11A compares PIC retrieved from MODIS AQUA, MODIS TERRA, and VIIRS SNPP satellites—time interpolated to the GP15 transect (Satellite Data), near-surface McLane pump sample values, and birefringent photon yield from sensors PIC011 and PIC010. The left and right axis scales are in the ratio of ∼15:1 PIC:photon yield. In the subarctic north Pacific, measured PIC exceeds both sensor and satellite PIC by factors of 2–3. From the 37°N subarctic front (station 12) to 11°N (station 21), measured PIC and the lowest values of satellite-retrieved PIC follow each other well (VIIRS SNPP data seem high). South of 10°N satellite and sensor trends diverge in trend with satellite PIC consistently low at the equator, while sample PIC registers a high. South of 10°S, satellite retrievals exceed sample PIC by a factor of 2–3.

Birefringent photon yield from both PIC011 and PIC010 matched satellite trends in the subarctic north Pacific (stations 3–10) but underestimated measured PIC. From the subarctic front into the north Pacific gyre (stations 12 and 14), sensor values exceeded sampled PIC by factors of 2–3. There was an excellent matchup at station 16 of all properties. From station 19 near 17°N to station 39 at 20°S, the sensors followed a trend of sampled PIC. PIC010 results generally are higher than those from PIC011.

Figure 11B shows the upper 500 m transect from PIC011 with additional detail of mixed layer depth, chlorophyll maximum depth, and estimated base of the euphotic zone. The subsurface birefringence maximum lies just above euphotic zone depths. Figure 11C shows 0–500 m transect data for PIC010 deployed during the GTC CTD profiling. The systematics of the section repeats Figure 11B but adds spatial resolution due to the more frequent sampling along the transect; PIC concentrations derived from PIC010 are generally 10–20 nM higher than values shown in Figure 11C. The minimum at station 21 is reproduced as is the relative minimum in the low oxygen waters at station 33.

We deployed PIC011, transmissometer CST1450, and Seapoint turbidity sensors during 90 CTD casts during the June 2017, California Current Ecosystem Long-Term Ecological Research (CCE-LTER) process study of an offshore-flowing biologically productive filament of recently upwelled water. Here, we focus on data from three spatial transects across the filament during different stages of its evolution (Figure 3). Transects T1, T2, and T3 were occupied on year days (January 1 = 1) 159, 169, and 174, respectively, and thus represent early to late views of the filament during its evolution. Figure 12A shows sections of PIC derived by multiplying birefringent photon yield by the 15.4 multiplier from the GEOTRACES midwater calibration relationship; Figure 12B shows POC derived from transmissometer beam attenuation coefficient—scaled by 27. Figure 12C demonstrates that birefringent photon yield is largely independent of the beam attenuation coefficient (and turbidity, data not shown).

The California Current regime is frequently cloud-covered. Temporal alignment of Transect T2 and satellites was excellent (Figure 3). Figure 13A shows PIC from VIIRS SNPP vs PIC estimated using surface layer values from PIC011. The agreement for satellite and PIC data is excellent over the first 30 km of the transect. Beyond this point, the sensor and retrieved data strongly diverge with satellite PIC dropping to less than 5% of the sensor estimate. The retrievals along the transect for days 168 and 169 (Figure 13B) and images in Figure 3 confirm the strong dropout of satellite-retrieved values. For comparison, Figures 13C,D show reasonable agreement between satellite POC and beam attenuation coefficient-derived POC estimates.

Figure 14 shows that the relationship between PIC measured in bottle samples collected from the upper 110 m during Transect T1 and birefringent photon yield at the same depth follows a 15:1 relationship suggesting that application of the GEOTRACES 15.4 factor is a reasonable approximation and, furthermore, that the dropouts of retrieved satellite data seen at transect T2 are real.

Figure 15 contrasts optical profile results from the Santa Barbara Basin during the CCE-LTER process study. CTD cast 2 was made into strongly oxygen-deficient waters where O₂ was less than 2 µM deeper than 500 m. Nitrate reduction was seen in waters deeper than 450 m. It is in this zone that the PIC optical proxy decreases with depth diverging from both beam attenuation coefficient and scattering which indicate increased particle concentrations in the water column. In contrast, the systematics of birefringent photon yield, beam attenuation coefficient, and scattering were in lockstep from 50 to 450 m. In shallow waters, the three sensors all showed increases into the euphotic zone but with different magnitudes.

The birefringence sensors show that PIC profiles vary spatially in the ocean far below the euphotic zone. We have documented the systematics of PIC distributions along the GP15 transect. The sensors resolved deep euphotic zone maxima of particle birefringence that almost always were missed by filtration-based sampling. We confirmed that elevated PIC concentrations penetrate to depths of thousands of meters, both near the equator and near the subarctic front in the north Pacific. These features arise from the fragmentation of sinking particles and thus are an indicator of export. We have seen clear indications of dissolution loss of PIC in strongly oxygen-deficient waters near 11°N and in similar conditions in the near-bottom waters of the Santa Barbara Basin. A detailed scientific synthesis will be reported separately.

The new sensors were tested in some of the most particle-free waters in the world’s ocean and are able to detect concentration changes to better than 10 nM. The profiles from the two sensors closely reproduced each other and were distinctly different from beam attenuation coefficient and scattering optical proxies for particle concentration. Below the surface layer and to within 1,000 m of the seafloor birefringent photon yield, PIC concentrations were highly correlated.

In the subarctic north Pacific, near-surface concentrations from the sensor underestimated measured PIC concentrations by factors of 2–3. This underestimation may be due to packaging effects of living coccolithophores. Typically, a coccolithophore has 20 coccoliths arranged in a spherical pattern around the cell. If the cell is living, then cellular contents block transmitted light from interacting with over half of the attached coccoliths. This may explain the underestimation of PIC from sensor data. This does not explain overestimation of PIC concentrations in transitional surface waters.

Alternatively, there are a multitude of coccolithophore species found along the transect (Okada and Honjo, 1973; Honjo and Okada, 1974). They were observed during the August 23 to October 8, 1969 R/V Hakuho Maru transect where 19 stations were occupied from 50°N to 15°S along 155°W. Honjo and Okada found that the coccolithophorid Emiliania huxleyi completely dominated the surface 100 m of the subarctic north Pacific; south of 40°N, the species assemblage became increasingly diverse reaching a peak in the north Pacific Gyre where E. huxleyi became a minor component of the assemblage. In the equatorial zone, E. huxleyi accounted for ∼30% of the population. In the deepest euphotic zone waters below 150 m, the species assemblage was dominated by Florisphaera profunda and Thorosphaera flabellata, and it is likely that these species may dominate the PIC maximum seen in deep euphotic zone waters in our section. For these reasons, there is also likely to be a factor of two species-dependent effects on the sensor response. There is justification for co-deployment of sensors in programs such as those described by Balch et al. (2016) where careful attention to counts of living coccolithophores vs detached coccoliths and routine measurements of the acid-labile scattering proxy for PIC are made. Alternatively, laboratory-based studies of sensor response to growing cultures of different coccolithophore species are possible.

Clay minerals are never abundant in the surface layer, except in waters recently impacted by dust. Particulate aluminum data from GP15 show little evidence of terrigenous material except near the seafloor or near continental margin sources. Other weakly birefringent particles if numerous enough may explain cases where sensors overestimate PIC compared to sample PIC in the euphotic zone. Acantharia (celestite) are rarely present in concentrations matching PIC; for example, Sr concentrations in surface waters of the Cape Basin were lower than 20 nM, compared with PIC levels exceeding 100 nM (Bishop et al., 1978). Scaled by the respective ratio of birefringence (0.009–0.172), celestite particles would contribute less than 1% to the photon yield signal. Similarly, barites with birefringence of 0.012 rarely exceed concentrations of 1 nM and contribute less than one part per thousand to the signal. This leaves some species of large dinoflagellates (e.g., Ceratium; Balch et al., 2007) to explain the observed differences. Our spike filtering of the data would remove the contributions of large organisms from the profile.

In the deep water column, we have detected birefringent clay minerals. Our aluminum data revealed that the sensor response is strongly dependent on the specific clay minerals in suspension. Illite clays which dominate in tropical waters have much higher birefringence than chlorite clays which dominate in the sediments of the subarctic north Pacific. The sensor reproduced these differences. Sutton et al. (2014) showed aluminum influence on the birefringent photon yield of our first-generation sensor (PIC001) deployed in 2012 and 2013 at Line P stations in close proximity to the continental shelf of Vancouver Island. Our results confirm these findings.

We have cross-compared satellite retrievals of PIC along the GP15 transect with near-surface sensor data. We found systematic agreement at stations from 55 to 11°N. At latitudes south of 11°N, the sensor PIC and sampled PIC covaried consistently, but the satellite retrievals became anticorrelated. In California coastal waters, we have shown that the satellite PIC retrieval scheme can strongly underestimate PIC by more than an order of magnitude compared with PIC measured in samples or estimated from sensor results.

During review, we were asked to comment on the new methodology described by Mitchell et al. (2017) who have updated the Balch et al. (2005) algorithm and also a new analysis of Terrats et al. (2020) who described a scheme to detect coccolithophore blooms using data from Bio-ARGO floats.

The Mitchell et al. (2017) scheme is in testing rather than production mode at NASA, and data are presently unavailable for our 2018 and 2017 study periods. We will compare the datasets when they become available.

Terrats et al. (2020) described a methodology that utilizes the ratio of scattering to beam attenuation coefficient, and alternatively, the ratio of scattering to chlorophyll fluorescence to detect coccolithophore blooms (PIC levels exceeding 1 µM) in surface waters using data from Bio Argo floats. The method capitalizes on the 15% higher refractive index (and scattering efficiency) of carbonate particles compared with phytoplankton. We have explored interrelationships among birefringence, scattering and beam attenuation coefficient, and chlorophyll fluorescence in euphotic zone waters sampled during CCE-LTER and found little sensitivity of the Terrats et al. (2020) proxies to PIC. Most likely, this is because surface waters were dominated by living phytoplankton and not detached coccoliths.

An advantage of our PIC sensor is that it discriminates against all non-birefringent particles and capitalizes on the fact that calcium carbonate birefringence exceeds other common biogenic minerals (SrSO₄ and barite) by more than an order of magnitude and the fact that carbonates are far more abundant than these other phases. We have further demonstrated that the sensor does detect birefringent clay minerals in the nepheloid layer. In other words, the sensor only responds to the birefringent components of the total material in suspension. We feel that there is strong justification for its further development and have demonstrated its value in the validation of remote sensing proxies for PIC and in extending knowledge of PIC in the vertical to full ocean depths. Further deployments during GEOTRACES expeditions are justified.

Two versions of our birefringence sensors have already been deployed on Carbon Explorer (Bishop et al., 2002) floats in 2003 and again in 2013. In the former case, the polarizing cell was lost from the float’s PIC sensor within days of deployment. The float and sensor electronics functioned for their complete lifetime. In 2013, two Carbon Explorer floats were deployed at station P16 along Line P with sensors very similar to PIC010 and PIC011 but fitted with fluid-filled pressure-compensated polarizing cells (Supplementary Figure S2). They had been successfully CTD profiled a month prior to deployment (Supplementary Figure S1), but as the ship transited to the deployment location, one of the cells leaked fluid due to ship motion, and attempts by personnel at sea to rectify the problem failed. One Carbon Explorer successfully collected 100 profiles, but fouling of protruding fixtures from the sensor led to bad data after 40 days. It was this difficulty that led to the current design of the sensors.

Our ongoing work will interface the current class of sensors to the Carbon Flux Explorer (Bishop et al., 2016; Bourne et al., 2021). This float already determines the birefringent photon yield of sinking particles as they accumulate on an imaging stage. Implementation of the PIC sensor will enable simultaneous measurement of birefringent particles present in the water column and the sinking flux of birefringent material. Further analysis of “spikes” in concentration profiles may provide insight into the occurrence of foraminifera, pteropods, and other birefringent organisms.

The birefringence-based optical proxy for PIC is new. We are still learning about it and working on how to improve its fidelity. Laboratory thermal cycling experiments demonstrate that the current generation of prototype sensors requires complex treatment of profile data. In our experiments, we were able to induce instrument temperature changes at rates from −0.6 to +0.6°C per minute. During water column profiling in the tropics, the sensors can experience rates of cooling or heating up to an order of magnitude greater than that achieved in the laboratory. Our first priority is to engineer the sensor to eliminate the hysteresis that can now be readily detected. The behavior of two co-manufactured sensors shows that thermal transients can generate leads and lags in the source and detector electronics. Our second priority is to compensate for variation of beam strength with temperature. We made a deliberate choice not to adopt the beam source monitoring approach used in commercial transmissometers until the sensor behavior at its basic level was understood. A third priority is to address sensor drift during operational deployments. During the GP15 transect, both sensors showed upward drift of their crossing blank. Furthermore, as they drifted, a pressure-dependent correction had to be applied to the profile data. It is not known whether the polarizers degraded or reacted with seawater over time, and this issue is under investigation. These three steps will transition the sensor from “specialist” to “routinely” deployable.

To conclude, this study demonstrates the potential value of a fast, readily deployable polarizing sensor that is capable of deployment from ship launched CTDs and autonomous floats. Expansion and enhancement of such autonomous observations of PIC profiles below the surface are critical for understanding and projecting the role of the biological carbon pump in the global carbon cycle.