Dataset: 14C and 13C measurements of particulate and sedimentary organic carbon from the Santa Clara River, California, during the 2023–2024 El Niño event

Sample Site Description

The SCR (Santa Clara River) is a SMR (small mountainous river) with a watershed area of 4232 km², (Henry et al. 2024; Swain et al. 2018). The river is in Ventura and Los Angeles Counties and 37% of the basin is controlled by dams (Brownlie and Taylor, 1981). Rainfall occurs in the winter, resulting in high streamflow from December to March, and dry periods during the rest of the year, typical of a Mediterranean climate. Sediment and POC samples were collected from the SCR after high flow periods from four precipitation events.

Samples were collected from the mouth of the Santa Clara River at three locations: 1) river water (for particulate organic carbon POC) from Santa Clara River Bridge, Highway 101, Oxnard, CA at 34.230, -119.259; 2) river silt (for sedimentary organic carbon SOC) at 34.230, -119.261, and 3) beach sand (SOC) at 34.230, -119.262. Bottle casts were conducted by hand from the Santa Clara River Bridge, Highway 101, CA (1 m depth) and sediment collection from the estuary during December 2023-April 2024. All samplings took place during rain events.

Particulate Matter Collection and Sample Processing

 All glass sampling equipment was soaked in 10% HCl for > 2 hours, rinsed with Milli-Q water, and baked at 540˚C for 2 hours. Metal sampling equipment was baked at 540˚C for 2 hours. Bottle and jar lids were lined with Teflon tape cleaned in Chromerge.

River water was collected by submerging a 2.5L glass bottle deployed by rope from the SCR Bridge. The samples were stored on ice for up to 24 hours until vacuum filtered in the lab using pre-combusted 2.2μm quartz fiber filters. The filters were frozen in glass petri dishes, wrapped in clean aluminum foil and plastic bags. For isotope analyses, filters were cut into sections using a razor blade cleaned with methanol, transferred into quartz combustion tubes, then acidified with 3% H₃PO₄ overnight and dried in vacuo.

Riverbank Sediment Collection and Processing

Sediment from the top 3 cm of the riverbank were collected in glass jars and stored on ice for up to 24 hours until transport to the lab, and frozen. Sub-samples were taken by thawing, and weighing approximately 200 mg of sediment in a petri dish before fumigation with concentrated HCl for 3 to 4 hours (Komada et al. 2008). Samples were transferred into quartz combustion tubes with quartz wool and dried in vacuo. Moisture content of sediment samples was determined by drying samples at 40˚C to a constant mass.

¹⁴C Analysis

Cupric oxide and silver were added to the quartz tubes, then vacuum sealed and combusted to CO₂ at 850˚C for 2 hours. The CO₂ was cryogenically purified and reduced to graphite using zinc reductant and iron catalyst (Xu et al. 2007; Walker and Xu 2019). ¹⁴C analyses were performed at the Keck Carbon Cycle AMS (KCCAMS) Laboratory at UCI by our group. Uncertainties for river POC and sediment OC samples was determined by pooled standard deviation of replicates (McNaught and Wilkinson, 1997). Uncertainty of river POC ∆¹⁴C values was ±4‰ and that for estuary sediment OC ∆¹⁴C values was ±45‰, and were high due to heterogeneity of the samples.

¹³C Analysis

 The CO₂ obtained from combusted samples for ¹⁴C analysis were split after equilibration for at least 2 minutes to prevent fractionation. The split was analyzed for δ¹³C using a Finnigan Delta Plus mass spectrometer at UCI. Uncertainty was determined using pooled standard deviation of replicates (McNaught and Wilkinson, 1997). River POC samples have an uncertainty of ± 0.1 ‰; estuary sediment OC samples have an uncertainty of ± 0.5 ‰ and were high due to heterogeneity of the samples.