This dataset contains the contents of 55 elements and grain-size distribution, determined for 1-cm sediment slices of one short gravity core and two box cores from pockmarks associated with submarine groundwater discharge at the Hanko cape in Finland, northern Baltic Sea. The cores were collected onboard the research vessel Geomari of the Geological Survey of Finland (GTK) during the SEAMOUNT Sampling 2019 campaign. Geochronology of the cores is constrained by the measurements of Cs-137 activity content. The box cores are composed of fine sand from the bottom of two pockmarks with active groundwater discharge, whereas the short gravity core captured organic-rich mud that blankets a currently inactive pockmark. The Cs-137 activity of untreated sediment slices was measured for 60 min, using a BrightSpec bMCA-USB pulse height analyser coupled to a well-type NaI(Tl) detector at the Geological Survey of Finland. No corrections were applied for the results because the aim was only to detect relative Cs-137 activity peaks. After the non-destructive Cs-137 analysis, sediment samples were freeze-dried, homogenized and halved, with one half analysed for multielement composition and the other for grain size distribution at the laboratory (Eurofins Labtium Ltd, Kuopio, Finland). The material for multielement analysis was sieved through a 63 μm mesh, and 0.2 g of the passed-through fraction was digested in a four-acid mixture of hydrofluoric acid, perchloric acid, hydrochloric acid and nitric acid (USGS Methods T01 and T20). After evaporation of the acids at 160 °C, the resulting gel was dissolved to 1 M nitric acid, and analysed for element concentrations by inductively coupled plasma-mass spectrometry (ICP-MS), or inductively coupled plasma-optical emission spectrometry (ICP-OES). Al, Ba, Be, Ca, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sc, Sr, Ti, V, Y, Zn, and Zr were analysed by ICP-OES, whereas Ag, As, Bi, Cd, Ce, Co, Dy, Er, Eu, Gd, Hf, Ho, La, Lu, Nb, Nd, Pr, Rb, Sb, Sm, Sn, Ta, Tb, Th, Tl, Tm, U, and Yb were analysed by ICP-MS. Commercial sediment reference materials and in-house standards were used for assessing measurement accuracy. Element concentrations for all reference materials measured with each sample batch fell well within ±10 % of the certified values. Mercury was measured separately for 0.2 g samples through thermal decomposition, amalgamation and atomic absorption spectrometry (US EPA Method 7473). Solid-phase contents of carbon and nitrogen in the samples were analyzed by thermal combustion elemental analysis. The pools of inorganic C and N are negligible in this setting, hence the total contents are considered equal to organic C and N. Sediment grain size distribution was determined for freeze-dried samples by wet-sieving through 20 mm, 6.3 mm, 2 mm, 0.63 mm, 0.2 mm and 0.063 mm ISO 3110/1 test sieves. The samples were pretreated with excess hydrogen peroxide to remove organic matter prior to the analysis. The <63 μm size fraction was further analyzed down to 0.6 μm using a Micromeritics Sedigraph III 5120 X-ray absorption sedimentation analyzer. The sieving results were merged with sedimentation data in Sedigraph software. Median grain size was calculated according to the geometric Folk and Ward (1957) graphical measures, as implemented in the Rysgran 2.1.0 package in the R 4.0.2 software environment. Clay is defined as grains finer than 2 μm, whereas mud is clay and silt (<63 μm), and sand is 63 μm to 2 mm.
