Ironing Out Fe Residence Time in the Dynamic Upper Ocean

Abstract

Although iron availability has been shown to limit ocean productivity and influence marine carbon cycling, the rates of processes driving iron's removal and retention in the upper ocean are poorly constrained. Using 234 Th-and sediment-trap data, most of which were collected through international GEOTRACES efforts, we perform an unprecedented observation-based assessment of iron export from and residence time in the upper ocean. The majority of these new residence time estimates for total iron in the surface ocean (0-250 m) fall between 10 and 100 days. The upper ocean residence time of dissolved iron, on the other hand, varies and cycles on sub-annual to annual timescales. Collectively, these residence times are shorter than previously thought, and the rates and timescales presented here will contribute to ongoing efforts to integrate iron into global biogeochemical models predicting climate and carbon dioxide sequestration in the ocean in the 21st century and beyond. Plain Language Summary Iron is a key micronutrient for organisms living in the upper ocean, and thus, its availability is one of the key factors controlling the removal of carbon dioxide via phytoplankton growth in much of the global ocean. Until very recently, measurements of internal iron cycling were scarce. This includes estimates of how much iron leaves the surface ocean via sinking particles. Due to the lack of observations, models struggle to reproduce observed patterns in global surface iron distributions. For the first time, we constrain the rate of iron loss from the upper ocean along three basin-wide transects and bring together all preexisting estimates to determine the timescales on which different forms of iron are retained in the upper ocean. Overall, our findings suggest that iron cycles more rapidly between the surface and the subsurface ocean than previously estimated, and we encourage the modeling community to utilize the wealth of data presented here to explore the global consequences of these findings.

Constraining global marine iron source and scavenging fluxes with GEOTRACES dissolved iron measurements in an ocean biogeochemical model

“…These high source fluxes need to be compensated by efficient scavenging and subsequent removal via burial in the sediments to reproduce the distribution and global mean inventory in DFe observations. This is in general agreement with observational studies focusing on the surface layer (Black et al, 2020;Sarthou et al, 2003). For example, Black et al (2020) estimated similar residence times throughout the global surface ocean (0-250 meters) for DFe ranging from approximately 1 month to 4 years depending on the region and specific iron pools considered, although noting that the uncertainties remain large (i.e.…”

“…This is in general agreement with observational studies focusing on the surface layer (Black et al, 2020;Sarthou et al, 2003). For example, Black et al (2020) estimated similar residence times throughout the global surface ocean (0-250 meters) for DFe ranging from approximately 1 month to 4 years depending on the region and specific iron pools considered, although noting that the uncertainties remain large (i.e. equal or greater than the absolute value of the estimate in each region).…”

“…As a result, the range of residence times estimated by the current global marine iron cycle models ranges from less than a decade to multiple centuries (Tagliabue et al, 2016), which limits our ability to confidently predict the impact of changes to the marine iron cycle on productivity in a future ocean. Observational estimates fall within a similar range (Johnson et al, 1997), noting that more recent studies estimate much shorter residence times in the upper ocean (~10 days -4 years) (Croot et al, 2004;Sarthou et al, 2003) depending on the dynamics, iron pools considered, and source inputs in different regions (Black et al, 2020).…”

“…normalized standard deviation values fall below the value one; Figure 7). High variance in the global dataset may not reflect mean climatological conditions simulated by the steady-state model results given the highly dynamic nature of DFe cycling particularly in the surface ocean (Black et al, 2020), and the limitations of spatial and temporal sparsity of the dataset. But note that the standard deviation was significantly improved in our best model simulation with variable ligands and high source/scavenging fluxes (#5 Atm+SedFeHigh_LigVar; see Figure 7).…”

Southern Ocean Upwelling and the Marine Iron Cycle

“…Additional uncertainty remains over the fate of this Fe even after it reaches the euphotic zone. A recent global compilation of flux calculations suggests that Fe undergoes very rapid scavenging (time scale <1 yr) in particle-rich surface waters (Black et al, 2020), further limiting its availability to phytoplankton.…”