15N and 13C natural abundance in suspended particulate organic matter from a Kuroshio warm-core ring

Abstract

Vertical profiles of the natural abundances of ¹⁵N and ¹³C in suspended particulate organic materials (POM) were determined together with PON and POC concentrations to the depth of ca 4000 m at five stations in and out of a warm-core ring (WCR).

The ¹⁵N and ¹³C profiles showed generally, with one exception for a ¹⁵N profile at the northern edge of the ring, the same trend: an increasing abundance of ¹⁵N and ¹³C in the subsurface layer, followed by a decrease with depth. At the northern station, the ¹⁵N abundance of PON in the euphotic zone was unexpectedly high (∼20.4‰). PON with high ¹⁵N abundance was also observed at 30, 75 and 150 m at the southern periphery of the ring. This was not seen for the ¹³C of POC. T-S analysis indicated that POM with high ¹⁵N content was transported from the northern periphery by the ‘cold streamer’ of the WCR.

In the depth zone of 100–500 m, where thermostad appeared in the ring, the ¹⁵N of PON increased with depth while the ¹³C of POC remained constant in the ring; conversely, the ¹³C of POC decreased with depth while ¹⁵N of PON remained constant out of the ring. Temporal variation was noted for the ¹⁵N abundance of PON in the thermostad of the WCR, but little variation was observed for the ¹³C of POC.

The spatial and temporal differences between the ¹⁵N and the ¹³C profiles were explained assuming an intensified vertical mixing of the water column in the thermostad of the WCR, and an order of magnitude difference between the abundance of refractory fractions in POC and PON enriched with both ¹⁵N and ¹³C. It is suggested that the POM ¹⁵N and ¹³C isotopie ratios provide complementary information to study particle dynamics in the ocean.