How is seawater chemistry responding to global change processes, and what are the consequences for marine ecosystems? How do advances in our understanding of chemical speciation give new mechanistic insight into biogeochemical processes? The thermodynamics of solution chemistry in addition to the distribution of trace metals and organic complexes along a gradient of particle sizes and ephemeral clusters are the focus of our session. Although solution thermodynamics is a relatively mature field, there is an upsurge of research on both measurement and modelling of chemical speciation and an increasing focus on the assessment of uncertainties. This session will also explore interactions between solid and soluble species, focusing on how recent developments examining the continuum between solid and dissolved (colloids, nanoparticles, etc.) alter our paradigm view of element cycling. The transport and reactivity of these ‘in-between’ species may differ from their fully dissolved or solid counterparts, yet they are the most difficult to sample and observe. We invite submissions pertaining to the measurement and modelling of chemical speciation; assessment of modelling uncertainties; abiotic and biotic transformations from the solid to soluble phase and back again; reactions at the solid-solution interface; trace metal binding environments; sediment-seawater relationships; and interpretation of paleo records.