Disentangling what controls the cadmium distribution in the Pacific Ocean

16 May 2023

Sieber and his colleagues (2023, see reference below) established the distribution of dissolved cadmium (Cd) concentrations and isotopes at 23 stations (represented as δ¹¹⁴Cd) along the GEOTRACES GP15 section, a meridional transect along 152°W from Alaska to Tahiti.

Their data reveal an important disparity between northern Cd-rich high-nutrient low-chlorophyll waters and Cd-depleted waters of the subtropical and equatorial Pacific. In the open ocean, a biogeochemical model simulates the data in Cd-depleted surface waters, with the lowest Cd concentrations influenced by atmospheric inputs of isotopically light Cd. Below the surface (and surprisingly even in the North Pacific Ocean), Cd parameter distributions are essentially controlled by Southern Ocean processes, water mass mixing and regeneration. Indeed, Cd-depleted Antarctic Intermediate Water has a far-reaching effect on North Pacific intermediate waters as far as 47°N, contrasting with northern-sourced Cd signatures in North Pacific Intermediate Water. Last but not least, this study reveals that a correlation between the North Pacific phosphate maximum and a negative Cd* signal at depth in the North Pacific Ocean reflects a regeneration signal of Cd and PO₄ at a slightly lower Cd:P ratio than the deep ocean ratio (0.35 mmol mol−1). This finding contradicts the hypothesis that this negative Cd* signal is due to in situ removal processes in low-oxygen waters.

**Figure 1.** The cruise track of the US GEOTRACES GP15 Pacific Meridional Transect from Alaska (St 1) to Tahiti (St 39), showing relevant surface currents.

**Figure 2.** Distribution of Cd* along GP15, overlain with oxygen contours (top) and phosphate contours (bottom). Note different depth scales. Cd* represents the deviation in samples from deep ocean Cd/P ratio, and is defined here as Cd* = [Cd] – ([PO4] x 0.35).

**Figure 3.** Dissolved Cd/phosphate relationship along GP15. Solid black lines represent slopes of data from this study (shallow slope = surface data, steeper slope = deep data). The dashed line represents a Cd* of 0 (global deep Cd/P relationship), where Cd* represents the deviation in samples from the deep ocean Cd/P ratio, and is defined as Cd* = [Cd] – ([PO4] x 0.35).

Reference:

Sieber, M., Lanning, N. T., Bunnell, Z. B., Bian, X., Yang, S., Marsay, C. M., Landing, W. M., Buck, C. S., Fitzsimmons, J. N., John, S. G., & Conway, T. M. (2023). Biological, Physical, and Atmospheric Controls on the Distribution of Cadmium and Its Isotopes in the Pacific Ocean. Global Biogeochemical Cycles, 37(2). Access the paper:10.1029/2022gb007441

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