Biogeochemical data from Marine Snow Catchers deployed in the epipelagic to mesopelagic zones of the Iceland Basin during a late spring bloom (May–June 2024, DY180)

The DY180 research cruise was the first of two expeditions conducted under the BIO-Carbon programme, which aims to reduce uncertainties surrounding the contribution of biological processes to long-term ocean carbon storage. The DY180 cruise took place in the central Iceland Basin between 27 May and 21 June 2024, with the primary objective of capturing the peak and subsequent export of particulate material associated with the late spring phytoplankton bloom. Three scientific projects contributed to the cruise: BIO-Carbon, PARTITRICS, and CHALKY, with a shared focus on quantifying the biological carbon and calcium carbonate pumps. This dataset contains concentrations of chlorophyll a (Chl a; analysed using a Turner Designs Trilogy fluorometer), biogenic silica (BSi; analysed with a QuAAtro 39 segmented flow autoanalyser), particulate organic carbon and nitrogen (POC, PON; analysed using a FlashSmartTM elemental analyser), and particulate inorganic carbon (PIC; analysed via a Thermo Element XR HR-ICP-MS). Samples were collected using four Marine Snow Catchers (MSCs), deployed at nine stations across the Iceland Basin, sampling from depths spanning the epipelagic to mesopelagic zones (15–750 m). Sampling was conducted across four MSC fractions: Tzero (instantaneous), Top (suspended), Base (slow-sinking), and Tray (fast-sinking). Replicate samples for POC and PON were analysed, allowing calculation of propagated analytical errors, which are provided within the dataset. No replicates were available for Chl a, BSi, or PIC measurements. Of the nine sampling stations, four were designated float stations (S1, S3, S4 and R5), which tracked gliders and profiling floats in a semi-Lagrangian fashion capturing the evolving bloom. The remaining five were roaming stations (R1, R2, R3, R4 and S5), selected based on satellite-detected coccolithophore abundance (note: S2 was not sampled due to bad weather). At float stations, vertical profiles were constructed from multiple depths, while roaming stations targeted two depths: one within the mixed layer and one below. This dataset provides key biogeochemical observations associated with coccolithophore bloom dynamics and carbon export in the Icelandic Basin, supporting improved understanding of spatial and depth-dependent variability in particle flux during the spring-summer seasonal transition.