Table:
| (in thousands of $) | 2019 | 2018 |
| --- | --- | --- |
| Oil derivative instrument (see note 24) | 45,640 | 84,730 |
| Operating lease right-of-use-assets (1) | 9,847 | — |
| Foreign exchange swap (see note 24) | 214 | — |
| Mark-to-market interest rate swaps valuation (see note 24) | 8 | 6,298 |
| Investment in OLT-O (2) | — | 7,347 |
| Other non-current assets (3) | 24,700 | 40,729 |
|  | 80,409 | 139,104 |
Paragraphs:
17. OTHER NON-CURRENT ASSETS
(1) Following the adoption of ASC 842, the balance sheet presents right-of-use-assets which mainly comprise of our office leases. This standard has been adopted under a modified retrospective transition approach as of January 1, 2019.
(2) Investment in OLT-O refers to our investment in an Italian incorporated unlisted company which is involved in the construction, development, operation and maintenance of a FSRU terminal to be situated off the Livorno coast of Italy, representing a 2.7% interest in OLT-O’s issued share capital. In May 2019, a major shareholder sold its shareholding which triggered a re-assessment of the carrying value of our investment in OLT-O. This resulted in an impairment charge of $7.3 million for the write down of the carrying value in our investment in OLT-O in the year ended December 31, 2019.
(3) "Other non-current assets" as of December 31, 2019 includes payments made for long lead items ordered in preparation for the conversion of the Viking into an FSRU. As of December 31, 2019 the aggregate carrying value of Viking long lead items was $16.2 million.
"Other non-current assets" as of December 31, 2018 was mainly comprised of payments made relating to long lead items ordered in preparation for the conversion of the Gimi into a FLNG vessel. Subsequent to the receipt of a Limited Notice to Proceed from BP in relation to the Greater Tortue Ahmeyim project in December 31, 2018, initial works of the FLNG conversion commenced in January 2019. Consequently, as of December 31, 2019, the aggregate carrying value of $31.0 million has been reclassified to "Asset under development" (see note 15).
Question:
What was the change in mark-to-market interest rate swaps valuation between 2018 and 2019?