Threat to thermohaline circulation via rising thermal diffusivity in microplastic-contaminated seawater – a dual-beam thermal lens study

The present study investigates the modifications in the thermal behaviour of sea water (SW) by microplastics (MPs) using the mode-mismatched dual-beam thermal lens (MDBTL) technique, as the uncontrolled use of plastics has led to MP contamination, posing risks to marine ecosystems and potentially disrupting thermohaline circulation by altering seawater temperature and salinity balances. The abundance of MPs in SW is revealed through their presence in the gastrointestinal tracts of fish, which are identified as polypropylene by Fourier transform infrared and Raman spectroscopy. Though the increase of salinity is found to lower the thermal diffusivity (D) of water, the increase of MPs is found to enhance the D value of SW. The study emulating MP contamination in SW revealed a D value enhancement of 30.71 % for 10 ppm of MPs. The increase in D implies faster thermal transport in seawater, which can disrupt the thermohaline circulation, a crucial component of global ocean circulation driven by temperature and salinity gradients. Dynamic light scattering analysis showed that at 10 ppm, MPs had a zeta potential of −15.54 mV and particle size of 1.952 µm, indicating moderate electrostatic stability; at 0.1 ppm, 50 nm particles with +1.19 mV zeta potential aggregated rapidly due to ion screening. The UV–visible spectroscopic study also points to the increase of UV absorption due to the rising amount of MPs, resulting in temperature rise. The rising D value of SW can seriously affect the marine ecosystem and the thermohaline circulation, transforming the local phenomenon into a global one, revealing the significance of the work.