Calibrating the transmissometer data from the Geotraces North Atlantic Transect

Christopher Hayes (Lamont-Doherty Earth Observatory of Columbia University)

The Geotraces North Atlantic Transect consisted of two legs: KN199-4 (aka GT10) and K204 (aka GT11). Transmissometer data was collected on three deployment systems during these cruises: (1) the Geotraces rosette or carousel (GT-C) which collected contamination prone elements in Go-Flo bottles, (2) a conventional Niskin bottle rosette operated by the Ocean Data Facility out of Scripps (ODF) and (3) a CTD attached to the end of the wire used for deploying in-situ pumps (PUMP). This document describes calibration of the raw voltages (from downcasts) recorded from the ODF and GT-C transmissometers and how they were calibrated into particle beam attenuation coefficients (Cp) which should be independent of the collection system. In this calibration the PUMP data was consulted but it will be submitted separately.

Equations used (from GEOTRACES protocols, 2010; see also Bishop and Wood, 2008):
[V_CTD_ref - V_CTD_dark ] = (V_factory_ref - V_factory_dark) *
                         (V_CTD_air - V_CTD_dark) / (V_factory_air - V_factory_dark)
       Transmission (tr.) = (V_obs - V_CTD_dark) / [V_CTD_ref - V_CTD_dark ]
                                          Cp = - 4 * ln(tr.)
V_factory_ref is the maximum voltage as reported by the factory
V_factory_dark is the minimum voltage (blocked path) as reported by the factory
V_factory_air is the voltage reading in clean air as reported by the factory
V_CTD_air is the voltage reading in clean air (cleaned windows) reported on board
V_CTD_dark is the minimum (blocked path) voltage reading reported on board
V_obs is the voltage reading observed during the cast
Cp is calculated here for a 25 cm path length transmissometer and is in units of [1/m].
V_CTD_air and V_CTD_dark were checked routinely at sea for the PUMP on both cruises and only occasionally on the second cruise for the GT-C and ODF. The transmissometer windows were cleaned routinely for the PUMP and GT-C on both cruises and periodically for the ODF system. Based on the relatively stability of voltages at the particle minimum (~3 km depth) for the PUMP and GT-C systems, there do not appear to be significant drift issues. This was not true for the ODF system.

The calibration equations calculations, using the shipboard voltage readings result in negative Cp values (>100% transmission) for both the GT-C and PUMP systems because the V_factory_ref is too low (presumably the distilled water used in factory calibration is more particle-rich than seawater). The maximum voltage for each system therefore had to be tuned (values listed below). The cruise-wide maximum transmission was set to about 0.997 which occurs in the deep western basin (cruise KN204 or GT11, station 12). The data from the first leg of the cruise (KN199-4 or GT10) was tuned so that the Cp profiles matched in deep water at the cross-over station (TENATSO = GT11 station 24 = GT10 station 12).

There was significant variability in the background (deep water) values on the ODF system. Either ODF had drift or windows weren’t always cleaned between casts. There was also a problem with the profile shape of the transmissometer data collected by the ODF cast. The maximum voltage on the ODF casts was reached at ~1km rather than 2-3km from PUMP and GT-C. I have attempted to correct the ODF data for drift by matching Cp values at the clear water minimum (~3 km depth) with the PUMP and GT-C casts, but the profile shape problem cannot be corrected for. I therefore do not recommend using the ODF transmissometer data quantitatively; although, for large beam attenuation signals like nepheloid layers or the hydrothermal plume, Cp estimated from the ODF casts is most likely a reasonable estimate.

The Cp results for the PUMP and GT-C casts generally appear internally consistent. The PUMP did not produce reliable voltage readings for the full water column at GT10 station 11 and GT11 stations 2 and 6. The GT-C casts also sampled demi-stations, shallow casts halfway between the full depth stations. One other factor about the data collection is that the during the GT-C downcasts, there was uninterrupted descent of the transmissometer (whereas during the PUMP up- and down- casts the transmissometer is stalled during deployment and retrieval of each pump from the wire). The GT-C rosette, however, did not sample at GT11 station 4, because of time constraints. This is unfortunate because this station was observed to have most particle rich nepheloid layer of the entire cruise. In order to have one file with Cp for all the deep stations, I have pasted in the Cp values measured by the PUMP system at GT11 station 4 into the GT-C data and did a slight adjustment so the deep background values matched between the surround stations 3 and 6 (GT11) in the GT-C casts.

References:
Geotraces. 2010. Sample and sample-handing protocols for GEOTRACES Cruises. In Standards and Intercalibration Committee [ed.]. http://www.obs-vlfr.fr/GEOTRACES/libraries/documents/Intercalibration/Cookbook.pdf.

Bishop, J.K.B., Wood, T.J., 2008. Particulate matter chemistry and dynamics in the twilight zone at VERTIGO ALOHA and K2 sites. Deep Sea Research Part I: Oceanographic Research Papers 55 (12), 1684-1706.

Calibration (tuned) voltages used:
            [V_CTD_ref - V_CTD_dark]    V_CTD_dark
GT11, GT-C casts:    4.686 V            0.055 V
GT10, GT-C casts:     4.646 V            0.055 V
GT11, PUMP casts:    4.852 V            0.055 V
GT10, PUMP casts:    4.8155 V            0.0556 V
GT11, st. 4 (GT-C)     4.8443    V        0.055 V*Note GT-C did not sample St. 4, these are the modified values for using the PUMP data at St. 4 in the GT-C section
For ODF casts:
Cruise    Station    Cast    V_CTD_ref-V_CTD_dark    V_CTD_dark
GT11    1    2    4.555    0.059
GT11    1    4    4.555    0.059
GT11    1    6    4.555    0.059
GT11    1    7    4.555    0.059
GT11    1    9    4.555    0.059
GT11    1    10    4.555    0.059
GT11    2    2    4.545    0.059
GT11    2    5    4.545    0.059
GT11    2    6    4.545    0.059
GT11    3    2    4.545    0.059
GT11    3    5    4.545    0.059
GT11    3    6    4.545    0.059
GT11    4    1    4.545    0.059
GT11    4    3    4.545    0.059
GT11    4    4    4.545    0.059
GT11    5    2    4.545    0.059
GT11    6    3    4.545    0.059
GT11    6    6    4.545    0.059
GT11    6    8    4.545    0.059
GT11    8    2    4.54    0.059
GT11    8    4    4.54    0.059
GT11    8    6    4.54    0.059
GT11    10    2    4.535    0.059
GT11    10    4    4.535    0.059
GT11    10    6    4.535    0.059
GT11    10    8    4.535    0.059
GT11    10    10    4.535    0.059
GT11    10    12    4.535    0.059
GT11    11    2    4.545    0.059
GT11    12    2    4.543    0.059
GT11    12    4    4.54    0.059
GT11    12    6    4.54    0.059
GT11    12    8    4.54    0.059
GT11    12    10    4.54    0.059
GT11    12    12    4.54    0.059
GT11    13    2    4.54    0.059
GT11    14    2    4.537    0.059
GT11    14    4    4.54    0.059
GT11    14    6    4.533    0.059
GT11    15    2    4.54    0.059
GT11    16    2    4.535    0.059
GT11    16    4    4.537    0.059
GT11    16    6    4.537    0.059
GT11    16    8    4.537    0.059
GT11    16    10    4.537    0.059
GT11    16    11    4.537    0.059
GT11    17    2    4.535    0.059
GT11    18    2    4.529    0.059
GT11    18    4    4.529    0.059
GT11    18    6    4.525    0.059
GT11    19    2    4.528    0.059
GT11    20    2    4.525    0.059
GT11    20    4    4.525    0.059
GT11    20    6    4.525    0.059
GT11    20    8    4.523    0.059
GT11    20    9    4.52    0.059
GT11    20    11    4.518    0.059
GT11    21    2    4.518    0.059
GT11    22    2    4.518    0.059
GT11    22    4    4.515    0.059
GT11    22    6    4.515    0.059
GT11    23    2    4.515    0.059
GT11    24    2    4.508    0.059
GT11    24    4    4.508    0.059
GT11    24    6    4.508    0.059
GT10    1    1    4.605    0.059
GT10    1    3    4.605    0.059
GT10    1    5    4.605    0.059
GT10    1    7    4.605    0.059
GT10    1    8    4.605    0.059
GT10    1    10    4.605    0.059
GT10    2    1    4.585    0.059
GT10    3    2    4.585    0.059
GT10    3    4    4.585    0.059
GT10    3    6    4.585    0.059
GT10    4    1    4.585    0.059
GT10    5    2    4.585    0.059
GT10    5    4    4.585    0.059
GT10    5    6    4.585    0.059
GT10    6    2    4.585    0.059
GT10    7    2    4.58    0.059
GT10    7    4    4.58    0.059
GT10    7    6    4.58    0.059
GT10    8    2    4.592    0.059
GT10    9    1    4.595    0.059
GT10    9    3    4.59    0.059
GT10    9    5    4.59    0.059
GT10    9    6    4.59    0.059
GT10    9    9    4.59    0.059
GT10    9    10    4.59    0.059
GT10    10    2    4.585    0.059
GT10    10    4    4.585    0.059
GT10    10    6    4.585    0.059
GT10    11    2    4.585    0.059
GT10    11    4    4.585    0.059
GT10    11    6    4.58    0.059
GT10    12    2    4.575    0.059
GT10    12    4    4.575    0.059
GT10    12    6    4.575    0.059

GT10-11_trans_Cp_ODF.csv