230Th normalization is a valuable paleoceanographic tool for reconstructing high‐resolution sediment fluxes during the late Pleistocene (last ~500,000 years). As its application has expanded to ever more diverse marine environments, the nuances of 230Th systematics, with regard to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of 230Th from across the global ocean at two time slices, the late Holocene (0–5,000 years ago, or 0–5 ka) and the Last Glacial Maximum (18.5–23.5 ka), and investigated the spatial structure of 230Th‐normalized mass fluxes. On a global scale, sedimentary mass fluxes were significantly higher during the Last Glacial Maximum (1.79–2.17 g/cm2kyr, 95% confidence) relative to the Holocene (1.48–1.68 g/cm2kyr, 95% confidence). We then examined the potential confounding influences of boundary scavenging, nepheloid layers, hydrothermal scavenging, size‐dependent sediment fractionation, and carbonate dissolution on the efficacy of 230Th as a constant flux proxy. Anomalous 230Th behavior is sometimes observed proximal to hydrothermal ridges and in continental margins where high particle fluxes and steep continental slopes can lead to the combined effects of boundary scavenging and nepheloid interference. Notwithstanding these limitations, we found that 230Th normalization is a robust tool for determining sediment mass accumulation rates in the majority of pelagic marine settings (>1,000 m water depth).

7The high particle reactivity of thorium has resulted in its widespread use in tracing pro-8 cesses impacting marine particles and their chemical constituents. The use of thorium isotopes 9 as tracers of particle dynamics, however, largely relies on our understanding of how the ele-10 ment scavenges onto particles. Here, we estimate apparent rate constants of Th adsorption (k 1 ), 11Th desorption (k 1 ), bulk particle degradation ( 1 ), and bulk particle sinking speed (w) along 12 the water column at 11 open-ocean stations occupied during the GEOTRACES North Atlantic 13 Section (GA03). First, we provide evidence that the budgets of Th isotopes and particles at 14 these stations appear to be generally dominated by sorption reactions, particle degradation, and 15 particle sinking. Rate parameters are then estimated by fitting a Th and particle cycling model 16 to data of dissolved and particulate 228,230,234 Th, 228 Ra, particle concentrations , and 234,238 U 17 estimates based on salinity, using a nonlinear programming technique. 18We find that the adsorption rate constant (k 1 ) generally decreases with depth across the and most depths. We clarify the conditions under which K/P is equivalent to the distribution 25 coefficient, K D , test that the conditions are met at the stations, and find that K/P decreases 26 with P , in line with a particle concentration effect (dK D /dP < 0). In contrast to the influence 27 of colloids as envisioned by the Brownian pumping hypothesis, we provide evidence that the 28 particle concentration effect arises from the joint effect of P on the rate constants for thorium 29 attachment to, and detachment from, particles. 30Keywords: GEOTRACES;Thorium;Particle Concentration Effect;single-particle class model;Inverse carbon. 46The use of thorium isotopes to trace particle dynamics in the North Atlantic has a rich history. parameters were large compared to the estimates themselves, and the authors concluded that the 66 data do not strongly constrain the particle cycling rate constants. The authors also estimated the 67 rate constant for Th adsorption normalized to particle concentration to be 5.0 ± 1.0 ⇥10 4 m 3 kg 1 68 yr 1 , and the rate constant for Th desorption to be k 1 = 3.1±1.5 yr 1 . In contrast to the other 69 model parameters, the rate constants for adsorption and desorption were assumed to be vertically 70 uniform in their analysis. ronment. These studies used 228,234 Th measured on particles intercepted by in-situ pumps outfitted 81 with 0.5-µm nominal pore size filters (Buesseler et al., 1992) to determine rate constants for Th 82 adsorption (Clegg and Whitfield, 1993), and for particle aggregation, disaggregation, and degrada-83 tion (Cochran et al., 1993;Murnane et al., 1996). A key finding of both Cochran et al. (1993) and 84 Murnane et al. (1996) was that, over the course of the bloom, the particle aggregation rate constant 85increased from about 0 to 30 yr 1 , while the particle disaggregation rate constant increased from 86 about 100 to 500 yr 1 . 87Mo...

Iron is the most important micronutrient in the ocean, but the nature and magnitude of its sources and sinks to the ocean are poorly constrained. Here we assess our understanding of the sources and sinks of iron in margin environments by synthesizing observations from the U.S. GEOTRACES GP16 Eastern Tropical Pacific Zonal Transect (EPZT) cruise near the Peru margin. GP16 observations showed elevated dissolved iron (dFe) concentrations along the margin, but a larger westward plume of dFe at slope depths (1000–3000 m) in oxygenated waters, rather than at shelf depths (100–300 m) in oxygen deficient waters. We examine the potential explanations for this unexpected observation. Multiple tracers from GP16 suggest that sediment resuspension was important at slope depths, which would lead to enhanced benthic flux of dFe above what was previously measured. The difference in the apparent persistence and penetration of shelf versus slope plumes of dFe into the interior of the ocean likely results from faster removal rates of the shelf dFe compared to slope dFe. The dFe sourced from the shelf was almost entirely in the dFe­(II) form, whereas dFe sourced from the slope was almost entirely in the dFe­(III) form. Although benthic dFe­(II) diffuses into oxygen deficient overlying waters, there is still oxidation of dFe­(II), which precipitates to particulate Fe­(III). In contrast, the slope plume appears to persist in a stabilized dFe­(III) form. We hypothesize that sediment porewaters with moderate organic carbon delivery to sediments and shallow oxygen penetration are especially good sources of persistent dFe to the water column.

14Thorium is a highly particle-reactive element that possesses different measurable radio-15 isotopes in seawater, with well-constrained production rates and very distinct half-lives. As a 16 result, Th has emerged as a key tracer for the cycling of marine particles and of their chemical 17 constituents, including particulate organic carbon. 18Here two different versions of a model of Th and particle cycling in the ocean are tested us- to the data errors. 28We find that model V2 displays a significantly better fit to the data than model V1. Thus, 29 the mere allowance of vertical variations in the rate parameters can lead to a significantly better 30 fit to the data, without the need to modify the structure or add any new processes to the model. 31To understand how the better fit is achieved we consider two parameters, K = k 1 /(k −1 +β −1 ) 32 and K/P , where k 1 is the adsorption rate constant, k −1 the desorption rate constant, β −1 the 33 remineralization rate constant, and P the particle concentration. We find that the rate constant 34 ratio K is large (≥0.2) in the upper 1000 m and decreases to a nearly uniform value of ca. 35 0.12 below 2000 m, implying that the specific rate at which Th attaches to particles relative 36 to that at which it is released from particles is higher in the upper ocean than in the deep 37 ocean. In contrast, K/P increases with depth below 500 m. The parameters K and K/P 38 display significant positive and negative monotonic relationship with P , respectively, which is 39 collectively consistent with a particle concentration effect.