The sharp ascent in dry bulk freight rates this week is getting mixed reviews from dry bulk shipbrokers. Available capesize bulker supply has tightened, particularly in the Atlantic Basin, amid more chartering out of Brazil. But more ships are expected to come to the market while cargo activity may slow.

The Baltic Exchange's assessment for average capesize spot rates reached $19,515 per day, a 108% jump up from levels at the beginning of November. Much of that increase has stemmed from the gains in the C8 Gibraltar-Hamburg trans-Atlantic voyage, which has gained some risen 159% to $24,909 per day.

The gain in freight rates is "demand-led and tonnage is tighter in the Atlantic Basin" one London-based capesize broker said. But he adds that rates could take a spill as it "does feel a bit frothy."

Some of the tightness in the Atlantic is attributed to more cargo activity from mining giant Vale, according to some market watchers. Vale is said to be shipping more iron ore to China amid a 28% one-month rise in landed iron ore prices.

Fixtures data show Vale has chartered two ships for the Tubarao-Qingdao voyage in December: Louis Dreyfus relet the 170,000-dwt Zhong May (built 2010) to Vale for $12.82 per tonne fio scale; and another unidentified capesize at $11.95 per tonne.

But the broker adds that Vale has not been more making inquiries than usual. Other than reported fixtures, it's been difficult to pin down other spot charterers that have been boosting rates.

One broker adds that cargo activity may be slowing. But more ships are expected to enter the market in the coming weeks as owners seek to restart idled vessels or speed up voyages.

Moreover, the oncoming rainy season in Brazil and Australia has the potential for causing port outages, which would slow capesize vessel demand.

Indeed, the regular seasonality in rates is expected to hit the market by January as China's buying slows down and more ships enter the market. Baltic's freight futures markets for the first quarter are pricing average capesize rates to be around $7,500 per day.

"Owners are more encouraged in this market," another broker said. But there's a "huge amount of hype" in the market currently.