Deseret News publishes conference minutes noting Pratt's announcement of plural marriage.
Deseret News, September 18, 1852, 2
Sunday, Aug. 29, 1852, 10 a.m.
Conference convened and opened as usual.
Elder Orson Pratt preached on the subject of marriage. The house was exceedingly crowded through the day; and in the afternoon, the Sacrament of the Lord's Supper was administered by the bishops, led by bishop Lorenzo D. Young: during which President B. Young addressed the assembly concerning the opening up of the last gospel dispensation, doctrine and ordinances of the gospel, modern revelation, &c., which was followed by the reading of the great "REVELATION, given to Joseph Smith, Nauvoo, July 12 1843, on MARRIAGE; and Conference adjourned to Oct. 6, 10 a.m. We have not room for the minutes of Conference, in full.
Although this news article does not mention it explicitly, the published sermons from the August 29, 1852, conference make it clear the "marriage" Orson Pratt was speaking about is plural or "celestial" marriage. This is further confirmed by Elder Pratt reading D&C 132 at the conference.